If I were in this situation, I would really try hard to fit two power amps so one drive only one pair of speakers. I understand if there is a turn table involved, it can be very expensive and it cost too much to duplicate two individual system. So just use one preamp driving two power amps. You control which speaker to drive by just turning on one power amp or the other.
Finding an amp that hav A/B speaker selector really limit the selection. You like tube amps, that make it even harder to find. With separate power amps, you open to a wide variety of good amps.....you might have those amps already and you don't even have to spend money buying a new one. You have a big room, there got to be somewhere you can fit an extra power amp.
You are very critical in listening, I just afraid you might not be happy if you put the limit.
Own designed power amp, own designed preamp, JM LAB Spectral 913.1 speakers, Rythmik F12SE sub.
Not hooked up: Nakamichi Stasis PA-7 power amp, Velodyne VA1210 sub, Kef Reference Series center, Kef Bookshelf speaker, Monitor Audio bookshelf speaker, Infinity rear speakers. Acurus 3X200W amp.