Leaving DAC on all the time OK?

Tubes last longer if turned off when not in use. The cathode has a limited lifetime where it is capable of full emission. There may be some turn-on stress, but that is outweighed by burning the life out of a tube.

On turn on, the filament is cold and conducts current with little resistance. The high in-rush current causes very rapid heating and thermal stress. Also, if the unit utilizes solid state rectification, the power applied to the plate come on almost immediately while the cathode may take some time warming up. This could lead to a condition called cathode stripping.

These issues are mostly non-existent if the unit uses tube rectification (the rugged rectifier takes all the abuse and slowly turns on the rest of the unit. In units with solid state rectification, the turn on can be rapid, but a lot of experts think that the "damage" is relatively minor.

It is possible to put a thermistor or other device in the power supply to slowly bring up the power in tube gear if that is a concern.

CDPs or DAC/Transport combinations draw very little power and usually don't put out much heat (high heat is what ages most components) so leaving them on is not a problem. With my CDP, if I turn it off, it can take several hours to get back on song. The manufacturer recommends leaving it on all the time. They claim that it takes quite a while for the capacitors to get back to optimum performance.

Raquel,

Good points, as far as general advice. All my tube gear has tube rectification so I don't worry as much about turn on issues.

I suppose the best thing to do is consult the manufacturer. Some linestages are quite hard on their tubes (old Counterpoints come to mind) and so turning them off might make sense, regardless of whether or not they use solid state rectification. Also, some gear run pretty hot (enclosed case, lots of tubes) so damage from heat could be an issue. Some caps and resistors don't stand up to constant exposure to heat.