PS Audio M700s or older Bryston 4bsst / 4bsst2 ?

I think it really depends on the vintage of the Brystons but rest assured that at sometime either sooner or later they will need recapped and that will be quite expensive.

How exactly do you know when they'll need to be recapped?  

When they start leaking and/or the SQ squares off. Distortion.

There must be someone you can borrow an amp from. I recently compared the Belles MB200 monoblocks to the PS M700 monoblocks and found the M700' to be superior in everything but giving off heat. I liked the Belles, but the PS had more dynamics and details.

((((Can I remove the power tubes from my SLI 80 and use the “subwoofer out” jacks to send the signal to my amp. That way I could have a remote-controlled preamp. koestner)))
Koestner,The Pre amp is way more important in driving amps I would suggest you get a real preamp with decent matching Gain ARC, Belles ETC. The Belles amps you listened to will wake up and should then easily convince you of their abilities and superb performance. Best JohnnyR