I received an answer to my last question elsewhere. Here is the answer, from Xman, if anyone else was curious:
"The older BP20's with the switch on the preamp are not upgradable. If you unit has the power switch on the power supply it can be upgraded. This is my understanding"
Unless I hear otherwise, that's good enough for me to work with.
Now - As for my set-up - like me, my system is old and peaked out sometime in the 1980s. Unlike me, my system has been very well maintained, and most all of my components have been fairly extensively upgraded, tweaked, and improved.
My system stared out as a stereo set, but I got into quadraphonic playback in the late 70s. My quad changed to a synthesized quad sound in the early 80s with a change to a Sansui QSD-1 and has stayed that way ever since. Bi-amping was added a short time later to help out my Magnepan speakers.
A couple of years ago I started going though my system and I upgraded almost all of the components. I've also tweaked the system with better tubes, fuses, interconnects, wall outlets, room treatments and power conditioners. I believe in getting the most amount of "bang-for-the-buck" out of my money; so, rather than spend $5,000 on a piece of equipment, I will spend $1,000 on a nice, older piece, add another $600 in upgrades, and figure I probably have almost all of the sonic improvement the $5,000 purchase would have provided. I retired as a musician Almost thirty years ago, but I still don't ever want to get to the point that I become more in love with my components than I am with the music.
I'm almost done with improving my system and when done - I'll just keep it this way. Unfortunately, my main preamp (the Audire) is broken. I suspect the Brystons I'm looking at will continue to provide the crystal clear details that my Audire did, with the added bonus of a somewhat deeper soundstage - which is something I really like.