Another perspective on this question can be found at amp designer David Belles' site "www.powermodules.com" (click on "articles" on the home page.) In the short article he states that both passive and active components can be subject to shortened life if the component is left on all the time. He feels that the best compromise is to turn the unit on a bit before the listening session to let it warm up.
OFF, Standby, or ON for amplifiers
I know that it is best to keep most digital gear "ON" all the time but how about amplifiers. I have been under the impression that if you never power down there is less stress on the equipment and promotes longer life. Is this true or am I way off? Thanks in advance...John