A new 3B SST 2 lists at $4,300 (and they almost never go on sale) and $3,000 used, and isn't a whole lot better than the SST you have. I see the 3B SST for $2,000 and $1,800 CDN so I'd say you pretty much paid market price if it is in very good shape. The ST is not the amp the SST is, But for the money is good enough, and likely a step up from a bunch of vintage amps. You should also get the remainder of the 20 year warranty. That alone buying used at these prices is a BIG bonus.
If you are looking for a good deal, You are looking at the wrong brand. Bryston amps, Like other high end amps aren't for people who want good deals, They are for those who want the most transparent sound, period. What ever you feed it, you get back in spades. Nothing more, nothing less. If you want want slamming bass, and concert level SPL's, that sound good, that can be done much cheaper. If you want to take it to the next level, Than Bryston is as good a way to go as any.
It should be noted however that unless the rest of your system is up to the SQ of Bryston, The improvement will be limited to your weakest link. If you can find a pair of Energy Pro 22 Monitor speakers that have had the tweeters serviced with fresh ferro fluid, and X-over caps replaced (that's the easy part) The speaker part of the equation will be done. I had my Pro 22's for 25 years before moving up to my Bryston amp from a decent NAD Preamp/Tuner and Audio Design power amp, and the difference is fine detail, Imaging and sound stage was significant. The backgrounds are so black with the Bryston it can be unsettling at first.
If you want a good matching Bryston Preamp, Try the BP 6 or BP 20. You can sometimes get a used B60 SST Integrated amp, which uses the BP 6 preamp for cheaper than the Preamp alone. Just make sure it has preamp outputs, I'm pretty sure it has.