I asked my guy at Parasound if I should power down my A21, and he said (paraphrasing) "the amp is warmed up in 30 seconds so even though Ma-Bell would like you to leave in on you should turn it off to conserve energy". He went on to say you can leave it if you like it wont harm the amp, but there will be no audible change (benefit) by leaving the amp powered on. I just ordered a new set of tubes for my CJ preamp, which was a few bucks. For me, the amp and pre are powered off.
I realize that my gear probably sounds better warmed up (30 minutes of play) however I turn it on sit down and listen.
My DAC is left on at all times and when my Mac is powered up the DAC awakes from stand-by mode.
Kirk