Reliabilty, Keeping Equipment Powered on or off?

I generally keep my system components powered on because the system does not have to "warm-up" when I sit down for a listen. I do know that reliability is usually calculated by POH (power on hours) however this calculation IMHO is a broad brush stroke. I do power down certain components that are not required to be left on to sound their best, such as a Luxman T-117 tuner and Wadia 171i Dock . However, my Krell IA, with the volume turned all the way down runs pretty hot, so hot you can't leave your hand on it. When turning the volume up on the Krell to reproduce the live event sound level, you can fry an egg on it, so I can only guess the Krell was designed to handle this kind of heat? However, back to the other components, such as DAC, SACDP, Headphone Amp, etc. Is keeping everything turned on asking for trouble or is leaving it on best to prevent thermal expansion and contraction which may be detrimental?