A couple of years ago I provided a detailed post here regarding this same subject which I can now no longer find.
I own a Bryston 4BST that at that time was a few years past the 20-year warranty. Bryston had an upgrade program in place for any generation amp to restore it to its original specs with new parts.
I took them up on this. It cost about $1,500 though there were several options for upgrades that could be selected at different price points.
My amp received extensive upgrades including all capacitors and output transistors. The amp was tested and burned in before it was returned with a new certificate of performance and three-year warranty. Bryston is known for repairing any amp of any generation at any time for any owner.
I am very satisfied with the performance of my amp finding it as transparent and resolving as before with all of the power that I need. The sound is consistently open and well defined, with no hint of noise, distortion, or compression. I also own Bryston's BP 25 pre-amp and use both with my Dynaudio S1.4 LE speakers. My listening tastes are mostly classical - symphonic, chamber, and opera - and jazz, mostly small group.
A few years ago, Soundstage Network published a review directly comparing the performance of the 4B3 to the 4BST. The 4BST acquitted itself well considering its age and it had not been reconditioned, but no, it's not a 4B3 and can't be made into one. At considerably less than the cost of the 4B3 though Bryston can and will make your 4BST continue to perform very well for years to come.
Good luck with your decision. You can't lose with either one.
