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.

In response to your initial question I would agree with Jond, leave the DAC and the transport on.

However, since the topic has gone a bit further into the value of warm-up from cold start vs a constant on condition I've made an observation with a SS CDP left on 24/7.

When the rest of the system has been operating well past any reasonable warm up point (all tubes) when I use this CDP (Wadia) initially the upper frequencies are definitely overbearing in the highs. Dissapears within about 30 minutes or less (replayed the disc to verify). Thereafter, in that session there is no problem whatsoever.

Anyone else observed this in a different, or same, devices?

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.

My Bel Canto dac doesn't even have an on/off switch. It's been going about 7 years now. It may be that your dac will last longer leaving it on all the time.

Tubes are the only issue here.

Larryi: It is precisely because most modern tube preamps use solid state rectification that powering them up and down kills tubes. I agree that a soft-start circuit (CAT preamps come to mind) or using a thermistor will do a lot to ameliorate the problem, but absent such a device, I stand by my statement that small-signal tubes last much longer if left powered up in most circuits. If a person is running a big Jadis or Aesthetix preamp with tube rectification, that's one thing, but most tube pre's (or anything else that uses small-signal tubes) do not, and 24/7 will yield better tube life (and to a point, better sonics -- as I have noted in other threads on this topic, they do degrade slowly over time).

The "TIPS & ADVICE" section of the owner's manual to my VAC Renaisance 140/140 Mk. III tube amps states:

"How long should tubes last? It has long been known in professional circles (and probably now forgotten) that a tube such as the 12AX7 will display BETTER performance characteristics after TWO YEARS of CONTINUAL operation than when it was new. In normal use it is not unusual for a low level tube to last 5 years or longer. Output tubes [i.e., power tubes used in tube power amps] are another story, as they are continuously providing significant amounts of current." (Emphasis original).