I have a AES tube amp and for kicks I hooked it up to the pre outs of the CA 640A v2 that I use in the TV room, and it really choked the life out of the tube amp. It is shocking, because the CA integrated is a darn good integrated amp. I wonder when you go up the CA food chain that they improve the preamp section besides adding more power.
I read your post concerning your 540A being used as a preamp. I have the Cambridge Audio 640 V2 which is an excellent (even above average) integrated amp. The preamp outs work beautifully for a subwoofer..but as a preamp into a tube power amp it was not very good.
I tried the pre outs feeding an Audio Electronics (Cary Audio) Superamp Mk2 tube amp, powering 94db Polk Audio RTA-11 speakers and it was not so stunning as it was as an integrated amp. There could be a lot of reasons but as a preamp feeding a tube poweramp was not as good "in my system" as a stand alone integrated amp.
I like your anti-Sullivan treatment of your system. Instead of "form follows function" you put form first. A true minimalist. Few have the nerve to install their flat panel off-centered, or choose/place speakers for their form in a 2-channel home theater system.
What I see of your living spaces looks beautiful. I like everything about it, including your equipment. You may be able to accomplish your pre/power needs w/ beautiful minimalist PS Audio products.