For $2500 used you can get a lot more amp than just a Bryston 4B-ST. Try a Plinius, Sim Audio, Pass Labs, Gamut, Blue Circle, etc. If you do purchase an amp and find out you don't like it in your system then you just turn around and sell it.
Now from personal experience, I own a 4B-ST, I have auditioned the 7B-ST in my system for a week, and I just purchased a Sim Audio Moon W-5. IMS the moon abolutely smokes the 7B-ST monoblocks in every way; dynamics, control, imaging, soundstage, transparency, top-end air, harmonics, etc. Note, that I am not saying anything bad about either of the Bryston's, just that the moon is better.
Now from personal experience, I own a 4B-ST, I have auditioned the 7B-ST in my system for a week, and I just purchased a Sim Audio Moon W-5. IMS the moon abolutely smokes the 7B-ST monoblocks in every way; dynamics, control, imaging, soundstage, transparency, top-end air, harmonics, etc. Note, that I am not saying anything bad about either of the Bryston's, just that the moon is better.