Do I NEVER shut it off…?

You might want to re-watch that one.

Frankly, my dear, etc, etc. 

Then watch A Streetcar Named Desire, where Blanch DuBois says, "Whoever you are I have always depended on the kindness of strangers."  

Pass Xa25.  Nelson says leave it on for an hour.  I have left it on for a week to experiment.  There is better space and transients.  Not worth the electricity.  Fyi.  Warm but not hot after that.  I turn off the tube preamp because i am under the assumption hours of life counts when turned on.

Whether a bad practice or not, i never leave something on forever. Heating and cooling cycles may cause strain but depends also on on construction and casing and how fast it cools down after shut off.

As I was told by a man with a great ear who owns several amps
including a Pass 250.8. Off, then a  One hour warmup. 

As I was told by another very experienced fellow also with a 
great ear. Keep the DACs on always. SS may take a week to 
fully wake up.

I go with the One hour warmup. Even for the DAC. After all there
must be some advantages to a limited hearing range.

As I was told by another very experienced fellow also with a
great ear. Keep the DACs on always. SS may take a week to
fully wake up.

I don't know about the ears but this one is a great listener. The other one, not so much.