I also must disagree with Pabelson's take on a couple of points:
"Tubes take a long time to settle in, so keeping them on makes some sense; for SS, just turn it on a few minutes before you start listening."
A) Tube gear requires less, not more, warm up time than solid-state to sound its best. The fact that tubes need a minute at the start and transistors don't is a red herring that I believe you're confusing with the more important aspect of how a piece of gear will sound over, say, a typical 2 hour listening session if started from cold. Tubes will be most of the way to their best after 15-20 minutes, but transistors will need at least that whole 2 hours, and preferably a whole day to really come into their own. Saying that you can flick on SS gear a couple of minutes before you listen and hear everything you paid for is naive, although if the piece of gear in question has a semi-powered standby state the difference may not be nearly as great.
B) In most cases it makes much less sense from a practical standpoint to leave tube gear always-on - you'll wear out your tubes several times quicker. And if we're comparing class-A/B power amps, then regarding energy usage ethics, the tube gear will draw much more power at idle than the SS.