proper way to a/b amps

The SS amp will be OK if you just leave it powered up, whether or not it's hooked up to the speakers, but the tubed amp will have to be turned off before being disconnected from the speakers. However, assuming both amps are warmed-up when you start the comparisons (do this by leaving both on for say 1 hour with the tubed amp connected and the SS amp disconnected - neither will be 'fully' warmed-up because you weren't playing music, but the fight should at least be decently fair), you should then be able to make reasonably valid comparisons as long as you don't go more than a couple minutes between switchovers, because the tubed amp will still 'stay on the boil' relatively speaking for a few minutes after being turned off each time. BTW, when I have done this kind of test, I've found that there is always some potential for fatigue engendering confusion about where I stand in the setup procedure if I get interrupted or distracted by something, so I make it a rule to always unplug the AC on the tube amp each time I disconnect it from the speakers, and replug it each time I reconnect it, thus preventing any accidental powering-up without the speaker load attached.

Geoff, One problem with powering both up is that most tube amps need to be connected to the speakers when they are powered up- be careful you don't damage the amp.

Identifying how amps differ in any aspect using a method of constant changing is fine, however if your not careful you might miss something detrimental in the sound because you are too focused. In any event, I recommend that after you do your short term comparison just relax and listen to one amp for a few days as much as you can, then take it out, plug in the other and see if you feel you are missing anything. I find this works best for me on most components and tweeks....its sort of an intuitive reaction but it usually stands the test of time.

yah I've tried the "long time" listening sessions...they just don't work for me...

I need to focus on a segment, and analyse one thing at a time...precision of soundstage...then width of soundstage, etc...my problem is I don't have the memory to compare over long periods of time.

If I power both units up for an hour.
then turn them off/on relatively quickly while I swap cables, will they maintain their heat enough to keep the "warmed up" sonic quality?

Yo,Jeffie; Best to turn everything off when switching amps and pre's. Everybody's hearing is different. For me I would listen to one setup for a full day say 6 hrs--- Then 2/3 hrs of the same, the next day. Then the other setup; likewise. AND leave things powered up overnight this is critical with ss amps. I remember driving myself crazy (saves on cab fare) trying to a/b two dacs.-- 15/20 mins. on each.(Yes, that's how I got this way.) I ended up buying neither. I just bought a different dac. (From a dealer with exchange option) --- About 2 mins. on the Theta Gen5, I knew "this" is what I was looking for. The other dac was the Res.Audio dac.---3rd being my Theta pro basic2.