Bryston or McIntosh which would you buy?

I currently have a McIntosh MC752 amp, and am considering an upgrade in the future. I know the 752 is not one of McIntosh's best amps. I love mac gear as I always wanted to own their equipment.

1) How much of an improvement will I see going from the 752 to say a 7270 or similar?

2) How would it compare to something like a Bryston 4B?

Please note that there is a good chance I will upgrade my speakers...looking strongly at Thiel. Current preamp tube based Aronov.

Why not use both and bi-amp your speakers if they can accommodate them. That is what I have. The MC275 on the middle and top of my VR4JR speakers and the Bryston 4BST for the bass units with a McIntosh C2200 as preamp it is easy to set up. They match perfectly.
I would upgrade within the Mac line (Autoformer coupled) because I prefer the sound to Bryston, which is also good. In your case, I would stay with Mac especially since you are using Thiel speakers, which can tend to be bright, which Mac isn't and Bryston can be.
There has been quite a few Thiel,Bryston demoes at audio shows
Even James Tanner(Bryston) uses Thiel speakers(among others)
with 2XSS-2Thiel subs.I'd audition them at home if you can.
It does seem like Bryston and Thiel mate together very good.

I don’t know the 752, but I have owned the MC7270 (two of them at the same time), MC2255, MC352, and the MC402… The 7270 is a great amp but it is a bit dated, and was bested by the newer MC352 and MC402. That said, I ended up replacing the MC402 with a VTL ST-150, and never looked back. It was less than half the price of what I sold the MC 402 for, and sounded far better in my system (driving Dynaudio Special 25’s).

You may want to rethink solid state, altogether.
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Ran a pair of Thiel 2.3s off of a Bryston 4b-st for many years, with various preamps in front, both tubed and SS (VTL, Rogue, Plinius) -- and liked it plenty. Never compared with a McIntosh, though, so no use there. Still actually own all of it, but currently the Bryston is bridged and driving the center channel (also Thiel, and cetainly overkill) and the mains are all Rowland-fed. Bryston makes nice amps, and you can't argue with that warranty. Also not familiar with the 4b (compared to the -st and the yet more recent -sst), but word has it they've gotten more refined with each iteration. Often a fan of picking up stuff a generation or two older, particularly with amps, as you geat some great deals that way. A used -st seems to be going for less than half of used -sst gear. Yes, please.