Hey man, love your records! I'm also obsessed with old techno and I work exclusively ITB (mostly FX based in a Basic Channel / Deepchord kinda way) so I've spent a LOT of timing looking for this kinda stuff.
So, the truth is that there aren't any plugins directly emulating the DP/4 or the Alesis reverbs. Audiority Xenoverb has a 'flow' algorithm which is based on the allpass architecture of the Alesis verbs but I don't think it sounds quite as gritty (
XenoVerb - Creative Multi Algorithm Reverb - Audiority | VST AU AAX). Other than that, ValhallaVintage
does sound incredible, but it almost sounds too expensive in a way since it's mostly based on expensive Lexicon, AMS and EMT units. I'm not sure if you can can make it sound
exactly like a DP/4 or Quadraverb but you can get it in a pretty good sort of general gritty direction. It takes some time to figure out though. The important parts are choosing the 80s mode, one of the more colourful algorithms (the 'dirty' and 'chaotic' ones are pretty dirty) cutting a good amount of lowend and highend, decreasing the stereo width and cranking the bass multiplier to around 2x-2,5x. Then, further processing can shape it really nicely, injecting some noise really helps since those old boxes were noisy as hell and then some saturation and tasteful bitcrushing (D16 Decimort and the new RX950 based on the S950 are pretty much the best at that). I would also check PSP 2445, it's based on the digital EMTs. It does a very good old digital plate reverb in an authentic way, not as thick and creamy as the Valhalla. If you add some saturation and EQ you should get pretty close to that 90s techno digital plate reverb sound. I prefer Valhalla because it's more flexible but the PSP does a really good job at that one thing.
As far as other FX go, the last 2-3 years have been really good in terms of vintage FX, altough most of them are based on analog FX, vintage digital ITB is still a bit lacking. First of all, check out everything from Audiority, the dude is on fire right now. As far as delays go, there were two great Space Echo emulations released in the last year or so, Audiothing Outer Space and Surreal Machines Modnetic. The new Bucket-500 from Fuse Audio does a really nice warm, gritty BBD delay, the best I've heard ITB so far. Audiority Deleight is based on the Korg DL8000R and does pretty good 90s rackmount multitap delay. If you don't care about realtime control, Acustica Audio Lemon has sampled like 20 different rackmount delays and it really nails THAT sound but due to the sample-based nature it's kinda unusable for dub-style live send FX. If you care to get into UAD, they have modelled the AMS reverb and the Korg SDD-3000. Cytomic The Drop does an incredible job at authentic analog filtering. XILS-LAB ChorX does a great gritty BBD chorus, it has a noise/hiss parameter that gets really dirty. For super gritty phasing Soundtoys Phasemistress is still king, actually all the Soundtoys stuff does cheap gritty vintage FX really well. Soundtoys PrimalTap is pretty much the best gritty vintage digital delay ITB out there. The Eventide H3000 plugins also do a very good job at warm vintage digital (especially the filters) and can get very crazy, probably the closest thing ITB to those old multi FX boxes if you program them right. Also check out the D16 FX, the phaser, chorus and flanger all sound great. Especially the flanger (Antresol) does a great job at emulating the Mistress pedal flange, which is on a lot of 90s records, I think. At last the ancient (in terms of plugins) MDE-X multi-fx from Korg are a bit of secret weapon for me. They're based on the Triton FX, I think and the modulation FX sound great. The delay and reverb are kinda ****ty, but in sort of an endearing, early 2000s multi-FX way.