How do you like the TT-800? I've been using a D800 for a bunch of years and we just got a Quilter 803 for the shop. I keep thinking the TT-800 probably has some cool tones with the tube pre.
The best way to describe it would be that it's a characterful musical amp that always adds something of itself to the sound.
Sticking with the Subway channel as the direct comparison, for (in my case very humble) playing and recording purposes, it makes for a richer bass than I had with the solid-state preamp.
You can EQ out the inherent scoop, which AFAIK is comparable to the 10-11 o'clock on the D series voicing, but the way the transients behave is 'warmer', less immediate and with a more pleasant texture.
In my limited experience, the Subway amps themselves are already softer in this way than other flagship solid-state preamps, so the Subway channel in the TT-800 takes that a step further.
Accordingly, if you're not finding the D800 to be too harsh or immediate (which it certainly isn't in an objective sense, IMHO) then I don't think there's a substantial benefit to upgrading to the Subway channel in the TT-800, especially with the price inflation which occurred in recent years. I got mine at the old price still, so while I foresee some flight-mandating relocation in my future, I'll likely keep the amp long into the foreseeable future. I would have planned the same with any other 800W Subway amp, they are all definitely Good Enough.
The Boogie channel is further into what I consider to be stereotypical tube territory, but I have no experience with the D400+ it's based on, or really any comparable flagship all-tube amps, to make any useful comparison beyond saying it's "more". I think gooey or 3D are often mentioned by other users.
The Quilter makes a whole lot of sense to me for pedal workshopping purposes, given the distinct voicings. IMvHO it suits the task far better than an instrument-in-and-of-itself amp such as the TT-800 would.