After several years of lurking around and learning from everyone here, I’m looking for advice on recording acoustic instruments… I want to use the mic preamps in my ancient Teac 2340 reel to reel machine. I’ll be using condenser mic’s so will go into a small preamp (Sound Devices) to get phantom power, then into the Teac, then into a Focusrite Clarett. My question is, does that seem reasonable? Any advice?

Why?

Anyone can try almost anything in audio to see if it does something they like. Apart from being careful to not blow up any equipment or cause electrical shocks to anyone, you should be cleared for takeoff.
Since your actual “small” mic preamp with phantom power will be between your mic and the 2340, the 2340 will be a hardware insert. I don’t know what the impedance matchups between those devices will be, and I would not assume that the electronics in a last century Teac are up to factory specs, but best case it will at least do something you like to some instrument or voice you put through.
And if it doesn’t do anything you love, no harm and no foul. Report back. I’m curious.

I'm putting my money on "does absolutely nothing but pass audio."

The mic inputs on the old Teacs were 1/4" and intended for high impedance dynamic mics so a condenser mic with a high output will easily overload them. You might like that sound or not but you should be aware of what you are working with there.

Back in the day there were XLR to 1/4" adapters that corrected for the impedance mis-match so you could use low impedance dynamics or electret condenser mics that were battery powered, usually AAA batteries or in some mics weird specialized batteries that could be tricky to source.

Most of these electret mics had output levels that were similar to dynamic mics. If you had something fancy like a Shure SM81 it was a bit hotter than a dynamic but still lower output than a large diaphragm mic but few people who were running a Teac had something like a Neumann or even an AKG 414 but if they did they knew how to use them.

If you use an external micpreamp the RCA inputs on these tape machines they were meant to handle consumer -10dbV levels so if you are running +4dbU (standard low impedance line level) you are going to be coming in about 10 - 20db too hot so be careful about your levels and maybe even consider using an in line pad unless you like the sound of really overloading the inputs.

Anyway, I hope my ancient history lesson here helps you out. If something doesn't quite make sense or you have other questions just ask.

I recorded a christmas medley 10+ years ago with acoustic guitar, organ, organ drum machine, bells, sax, electric guitar, and tenor male/alto female vox through a variety of cheapo mics into a Roberts 770 as preamp into an old Tascam firewire interface. Kind of a rad gel. Will dm you the track.

Mostly I use reel to reels as guitar amps these days, give that a try on your Teac. Some things will sound better cranked than others. For more control is using it to blow out drums/guitars if you have a mixing board you can set it up as a parallel effect before hitting your interface.

Anything is "reasonable" if you are going for an effect. The other day I posted about these guitar amps that are made from the audio section of old 16mm film projectors, the kind that schools used to have. The sheer inappropriateness of this repurposing is part of the grooviness of the sound.

These tape deck preamps were generally so cheaply made that they overloaded very quickly. This makes them sometimes popular today for deliberate overloading and you might get some interesting sounds that way. If your goal is abuse, then give it a try. Why not?

Using your Sound Devices preamp for phantom power will provide only a line out, which even for' abuse' might be too much for the Teac's mic preamp. Get something like this for phantom-only. It has a mic-level out. You will still need the hi-Z adapter like this.

OTOH, if by 'reasonable' you expect a 'good' useful every day sound out it, i.e. better than your interface preamps or whatever, well... just don't kid yourself that you are getting a "vintage Neve on the cheap" here.

The transformer adapter is certainly a valid way into the Teac (it was the way many did it back in the day if they didn't have a mixer) but using the Sound Devices micpre along with a pad into the Teac's line input would be the technically "better" signal path.

The Sound Devices micpre is a good one and it also has a nice limiter in it. Seems a shame to not use this as the OP already owns it but it's a toss up whether it into the line input or instead going with the transformer adapter and smashing the Teac's mic input would be more the sound that the OP is after.

As I said earlier the Teac's mic inputs were really intended for dynamic mics not a hot large diaphragm condenser microphone so abusing them might be the charm here... or maybe not.

As the character of Merlin said in the 1980s movie Excalibur: "Looking at the cake is like looking at the future, until you've tasted it what do you really know? And then, of course, it's too late"