I have owned a lot of Bryston gear, including the BP20 preamp. I would take the incredibly transparent Benchmark LA4 preamp any day over the BP20. I think it may be cheaper too. I would use the LA4 with any priced gear but would never consider that with the BP20.
I am now using the LA4 with a CODA #16 amp and it is killer. Today, I upgraded my WyWire Platinum speaker cable to the WyWire Diamond and the LA4 will let you hear the difference easily.
Edit: Oops, I see you asked about the BR20. I never heard that one. I will leave this post up in case it is useful.