I agree that for sound quality, it is best to leave most gear powered up. This is particularly the case with solid state stuff which takes a long time to stabilize. As far as longevity is concerned, I don't think gear should be left on. It certainly is not the case with tube gear and not the case with solid state gear that runs hot when idling (biased high into Class A) -- heat is a killer of components.
Tuners and other low temperature, low power dissipation gear probably is not hurt much by being powered up all the time (aside from greater vulnerability to power surges/lightning strikes). My Dynalab tuner is actually on all of the time, even when the power switch is turned off (this only mutes the output and turns off the panel lighting). Dynalab apparently favors being powered up all the time. My CD player is also designed to be powered up constantly. My solid state preamp, a Levinson No. 32, is also designed to be on all of the time.
Tuners and other low temperature, low power dissipation gear probably is not hurt much by being powered up all the time (aside from greater vulnerability to power surges/lightning strikes). My Dynalab tuner is actually on all of the time, even when the power switch is turned off (this only mutes the output and turns off the panel lighting). Dynalab apparently favors being powered up all the time. My CD player is also designed to be powered up constantly. My solid state preamp, a Levinson No. 32, is also designed to be on all of the time.