TL;DR version:

If you had one analogue synthesizer to focus on, and chose a Behringer System 55 as that synthesizer, what would you add to it to expand its abilities, either out of Behringer's cheap modules, or aspirational fancy things?

Context/long version:

30 years ago I enjoyed playing music, bought synths relatively infrequently, and played them. I had a good turnover of models, but there was a core of ones I liked and I enjoyed. Computers couldn't really do the "VST" thing well, and it was all new to me anyway plus old synths were cheap, particularly as I funded my hobby by buying broken ones to repair.

At some point i lost this. Wanting to record what I was doing added a layer my ADHD-addled (as it turns out) brain couldn't handle because I used to layer things and play 'live' with effects and everything, and when I wanted to record I wanted each track separate etc.

It's got worse though, because rebuilding my setup, I've fallen into the trap of kit being pretty cheap, wanting to write about it and being distracted by all the new releases, PLUS the plugins got good so I have Arturia, Roland and Korg plus others than effectively give me every synth I loved and used 'in the box',

So with six analogue synths, mostly 2 VCO without patches, I wasn't being compelled to make them do anything to play music, only to play with synths. I couldn't make anything better with my Odyssey than I could with the presets on a virtual Odyssey. I barely touched my 2600 because the small controls and my eyesight don't play ball together.

I'm selling as many as I can. Things like the POM-400 would have been wonderful if that's all I had, but in a pile of stuff and my limited ability they are just toys.

The end setup will be:

Computer with Apollo silverface plus 8200 ADAT - right now the 8200 is used as effects send/returns linked to Korg KP3+ and Kaossilator Pro+, Ocean Machine II, and a spare that was being used for a Nineteen multi-FX.

Korg MW2408 mixer. The buses for the mixer go to the 8 line inputs on the Apollo.

Akai MPC Key 37 connected to Korg mixer over USB. This means if I want to play and not involve Cubase, I switch on the mixer and its speakers, the Akai audio goes over USB and can be assigned to groups if I'm using Cubase and the Apollo. It also means i can use the MPC as a sampler for anything I'm playing on hardware stuff - it's very easy to use as a looper.

Polyend Synth on the desk next to the MPC Key. I'm putting this into the MW2408's mono channels so I can use the built-in compressor but I still can't get the pads to respond in quite the way I want on presets, which means I'm not sure if I could make my own sounds work in a playable way.

Korg Taktile 25 on the opposite side - I got this as a cheap DAW controller, the touchpad is good for iWavestation and I like having transport controls.

On the next desk, Hydrasynth keyboard, Argon8 keyboard, Elektron model: samples, and in current thinking a PWM Malevolent, plus the System 55. A patchbay with aux send and returns from the Korg, a 16-channel line mixer if needed, and that's that.

I've got an XKey 37 LE (which drops neatly over the MPC's keyboard) so between the three keyboards and the Polyend Synth's pads, I can control most things when sitting at the computer, but I can also pick up the XKey and use it with the System 55.

Thoughts and improvements over this are "do I upgrade the Argon8 to a Carbon8, or even sell the Argon8 keyboard and get a rackmount one" "should I sell the ProVS Mini and get the modular version", "Do I need to keep the IK Uno Pro because it's very nice sounding and easy to mess with" and "I was thinking I'd sell the Hydrasynth and upgrade to a Deluxe, but for the difference in price I could get a desktop".

I still have a JV-50 with VE-JV1 expansion (so, a sort of cut-down JV-880 I think), so I think the only gap in physical hardware is an analogue polysynth, which I guess could be filled by a Behringer Pro800 or UB-Xa, but I'd pretty much decided the only keyboard I still really want is the cloned PPG Wave - that was a dream synth.

(If I wanted to be true to where my heart was when I first played synths I'd get a Roland D-05 and the little keyboard for it to live in - or just find space for a D-50 again. But I have the Cloud version).

I used to like playing big ambient and electronic things, dreamed of being able to do stuff like Delerium or what is now 'synthwave', most of that is in VSTs anyway these days. So I think I just want to get sequenced patterns and play pads and stuff and just enjoy myself again, stop thinking about 'what\s best' or trying everything.

I enjoyed, a bit, the feeling of having a stupid amount of gear, etc. - but ultimately that's a huge waste of time and resources if I don't make anything with or of it. I get more satisfaction focusing my attention on car restoration and repair if I want to do tedious/physical/thinking work - I'd like the music to be 'relaxing' rather than 'comparing synths and WANTING MORE'.

I am in a similar situation, only I already bought a Behringer System 55 and "pimped" it up with a few modules already. And also have a system 100m, 2600 and 2500 + 8 behringer monosynths to go with it. Along with the Pro-800, Ub-xa and Wave you mentioned. =)

At this point of my journey I could suggest two different roads to take than I did. I also have the same issue of retaining full control of every aspect of the song I'm making and this has been a road to ruin as far as my ability to have my songs finished. Too many variables, too much to worry about, no recall which means that as long as I am invested on a song under progress, all of my gear is unusable for other purposes as I would loose the settings if I do something else.

So for the two roads... Try picking one synth. No matter which, but ditch all the rest of them from the active setup. Drag them to storage, disconnect them or just mentally restrict yourself. Then make a song using only that one synthesizer. After each sound you record it to audio. And you build the whole song like that. Do not save the settings, but aim for the sound that you are happy with from the getgo. This forces you to commit, and quickly makes a habit out of it. Expecially when you deside to change things and are forced to dial in a sound that's similar to what you had in the part you decided to change. You will learn that the sound is something you create, and it is not in any way important to save it for later recall or reuse. The synth will became an instrument you play to get the sound you want. Continue this habit and change the synth from time to time to thoroughly learn how each of your existing synths works, and in what kind of sounds it excels at. After you have gone through them all this way you should have a good understanding of strengths and weaknesses of them all AND you have internalized the idea of making them sound the way you want as you go. You will also have a habit of committing to audio and focusing on getting stuff down ready to be mixed.

The other way forward is to completely rid yourself from hardware and focus on completing the songs using just plugins. You can also decide to do the plugin versions first and substitute plugins with hardware synths after the track is finished in it's plugin form.

These are both roads I personally consider at this point. It's fun to have all the toys you could possibly want, but it is impossible to play with all of them, expecially when you are afraid to commit on what you do.

Tinkering with hardware can be a lot of fun, and give you fresh angle to your music making. It can also kill your artistic output completely.

As it comes to system 55 alone... I don't see it so much as a versatile synth to have as a sound design tool. It is a couple of monosynths and a sequencer you can (have to) patch to your liking. It is also missing a lot of features you can have with monosynths costing way less, namely portamento, sample & hold and a separate and versatile lfo generator (you can use the oscs for that, but an lfo would be less costly and more user friendly option).
As a piece of legendary synth history recreated it is an awesome thing to have, and I tend to get stuff out of it I don't come up with my other gear. But for me it's more of a loop tool as in I tend to make things that are based on it's sequencer and come up with patches that live and breathe on their own rather than stuff that I play as a sound in a song. Of course it can be used like that as well, but somehow I always end up adding the sequencer to modulate things if nothing else.

Well, the System 55 arrived, and I've already spent more time messing with the layout than the actual synth!

First - added a 1630 Bode Frequency Shifter because I'd got Andertons points I could spend.
Second - sold my ProVS Mini and bought a Victor
Third - bought a Brains - I wasn't sure, but it was cheap and I am fascinated by how many of these modern synths, like MicroFreak, Polyend Synth, etc. all seem to be running on the back of the same sort fundamental building blocks and algorithms. So I'm curious about what I can do with it in the modular environment.

It takes a lot of time to reorganise and damn Behringer needs to sell the blanking plates, or include them. I also don't get the point of the bus with more pins than just power, those little ribbon cables could be carrying CV, gate on Behringer modules just as easily as other Eurorack stuff?

That would at least replace some of the stuff the original Moog does in terms of patching chaos, but then I don't know why the cases go the trouble of putting a power input and supply and a switch, but not side/rear mounted TRS sockets etc.

But I guess as a case to buy, most users may not want the USB/MIDI thing.

I haven't calibrated yet, and I need WAY more short cables so I can do things like linking the oscillators, but I love the sequencer to use, and am having fun with the choice of filters - I might get the AMSynths modules at some point to give it some upgrades if i make proper use of it. On the other hand, it's really teaching me a lot...

...about why bog-standard analogue modular is really a bit crap 'as a synthesizer' given the 50 years of progress or so. And of course when someone buys a Moog System 55 they have a: a crafted bespoke classic instrument and b: a way of REALLY showing off how much money they've got.

The Behringer strips away all that wealth and snobbery and you just question why you're snaggling cables over and around to simply connect an envelope to a VCA. On the other hand, I am fully aware that if I knew MORE about modular and had slightly more budget - or knew more about Behringer modules - it's possible to get much better, easier to use things to get the building blocks. The System 55 is super-raw to replicate an almost prototypical era of synthesizers accurately and I'm enjoying that aspect of it, but damn, I would not recommend it to most people.

An interesting side effect - my original plan was SELL ALL THE SEMI-MODULAR STUFF I DIDN'T USE WELL. Naturally I've been browsing for modules, mostly because I wanted a convenient patch panel and was trying to find a clever way to distribute signals a bit - and you very rapidly realise how cheap Behringer's modules are against the rest of the market, and how weirdly some modules are popular, mass produced and expensive, then you find something hand soldered by someone offering a webshop with kits and it's remarkably reasonable, and there's so much overlap of components and modules that I almost want to go back to "Korg vs. Roland".

I wanted a ring modulator and some sort of effects, like a spring reverb, to replace a couple of functions I did use on my B2600 blue. And as suggested, the lack of S&H and LFO was noted right away.

Well, the B2600 is out of the box and back on the desk, because with new prices under £300 selling a nearly four-year-old one for any reasonable saving over new just isn't worth doing. I could flog it for a decent discount to get the sale, and blow the lot on two buffered mults and an envelope generator. I'll keep it advertised, but I don't expect it'll go, and I'm using it alongside the System 55.

I'd already been tempted by the cheap 2600 VCO module, though AMSynths' 2500 ones are probably a better buy if I wanted to do that. But I feel like I need a CV-controlled mixer rather than yet more oscillators. The 921A/B combo soon uses up the three mixers provided.

The rest of the plan though, that's gone well so far. I've actually sold or put up for sale 11 synths, though I decided to keep the Elektron Analog Keys and sell the Argon8 (again) because it's DSP in a box with keys vs. DSP in THE box. Likewise I've kept my MS-1 Mk2 because it's just simple and easy, and right now the BassStation Rack has stayed because I'm feeding it through the MS-1 and having a lot of fun with that and creating patches for the BS Rack that are designed to complement an MS-1 patch and the filter/FM mod. 1VCO + 2DCO fun. I haven't tried going the other way and feeding the MS-1 out to the BS Rack input yet, but I also have a Crave lurking that I'd kept to use as a sequencer for the System 55 (though, I think the Elektron AK will do that for me).

Getting rid of everything else (and I do mean everything, CraftSynth, Skulpt SE, they are all boxed up and about to be listed) meant this weekend I focused on just playing the MS-1, Hydrasynth and messing with the MPC Keys. More 'playing music' than I have in ages. The only synth that's making me think a bit is the Polyend Synth - I really liked it when it came out but the more i use it, the more I'm annoyed by the grid response and so forth. But not as annoyed as I am by the used prices for it - a £200 loss (I've seen them for sale for £250ish remarkably often) in a few months of waiting for the firmware to fix a few bugs is pretty dreadful. I'll stick with it for now but it's not adding much to what I'm doing.