Digital gear takes several hours to stabilize in most cases. If possible leave this on. In most cases, it draws little to no power, even when running. Tube based DAC's obviously pull more power than most SS DAC's, but the added expense would probably not be noticeable.
SS line level components "should" come up to operating temperature rather rapidly and sound close to optimum within a relatively short period of time. I'm talking less than a few short hours here. This can vary from product to product though. A few of my preamps can not be turned off even though they have power switches on them. Those switches simply energize the accessory power outlets on the back of the preamp. Obviously, the designer knew what he wanted for his circuitry and designed that feature into the product. Once again, take into account that these components typically draw very little power and the effects of leaving them on would probably not be noticed on your bill.
SS amplifiers will vary with appr warm up time due to biasing levels. The higher an amp is biased and the hotter it runs, the longer it will take to sound good when going from stone cold to operating temperature. Even after an amp has come up to operating temperature, it will still undergo mild fluctuations for a period of time. As such, amps that run anywhere from cool to room temperature while operating ( like digital amps making use of switching power supplies ) will sound closer to optimum performance right off the bat than an amp that is Class A or richly biased into AB when first turned on. High bias amps tend to sound best after being on for extended periods of time in my experience. If you are going to leave one of these amps on though, you BETTER make sure that it have VERY adequate ventilation.
Tube products need to be judged on an individual basis. I think that most tube based power amps should probably be turned off or put into "standby" or "idle" when not in use. Turning them on and letting them "warm up" before being put to use is recommended. Since output tubes tend to run pretty warm, they should reach operating temperature relatively quickly. To help prolong tube life, one should let them "heat down" after heavy use before killing power. This is done for the same reason that you should let them "warm up" before applying any form of major signal to them. This is EXTREMELY important with devices that mount tubes horizontally and will help prevent premature sagging of the plates.
Line level tube gear is an "iffy" situation. Since the tubes are not "cooking" themselves to death nor drawing major amounts of power like an output tube, some people like to let them idle. Others want to save the tubes in terms of eating up their operational hours of life, so they turn them on as needed. Only trial and error will tell you what works best with each component. They should at least be put into "standby" mode if not being operated if you don't turn them off and on as needed.
My basic beliefs are that most devices last longer and sound better when being left on all the time. Most of the electronic equipment that is see come in for repair goes dead during "off time" or at "fire up". As far as i'm concerned, "in-rush current" and temperature variations take a HEAVY toll on electrical devices. Personally, all of my SS gear stays on 24 hours a day and my tube based system is energized as needed. Honestly, i don't use the tubes that often so there would be no sense in letting them run for nothing. I haven't gotten that system to where i want it to be, but i am working on it. Besides that, i have more fear of tube gear creating a fire hazard while unattended than i do of SS gear doing the same. THAT is something to think about. Sean
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PS... These are strictly my opinions, so take them for what they are worth. Since they aren't even printed on paper, they may not be worth that : )
