Thanks to the help of a fellow redditor, I was able to contact Bryston service. The person I contacted sent me a pdf with instructions for the process at rma.bryston.com. The instructions were unnecessary because the page is straightforward.

I also learned that the US repair center is closed, so all repairs are being done in Canada.

Coincidentally, I received a call from a tech today, following up on my voicemail from Monday. I was told that all out of warranty repairs are being done at a flat rate of $600 or you can trade up to new equipment with a 40% discount. My 30+ year old preamp and amp sell used for less than $600 each (maybe total!). I would say I got good value out of them, so I’m considering the trade up path. But that’s still a lot of coin! Lol

I tried the two local repair centers and neither of them wants to work on Bryston gear. Another kind redditor suggested an Atlanta repair tech that starts at $120 for the first hour, plus parts. I’m still considering that route because I still enjoy the equipment I have. My only hesitation is that given their age, other parts are bound to start failing.

TL;DR Bryston is now returning calls, doing all repairs in Canada, and charging $600 flat rate. An Atlanta tech is much more reasonably priced.