The mfr of my preamp (Pass) states in the manual that it takes at least 24 hours warm-up time before critical listening and I’ve found that to be absolutely the case. Recently unplugged it for a brief time and I had to start all over again. It’s not a difference that’s hard to hear.
Anyone out there know why this would be?
Some (including you): It’s true!
Others: It’s BS
Some: It takes time for the components to stabilize and warm to optimum operating temperature.
Others: That only takes a few minutes.
That pretty much covers it. 
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My own experience is that digital equipment unplugged and left to cool can take 24 to 48 hours to sound its very best. Don’t ask me why, I don’t have any convincing arguments that would satisfy skeptics. It just is what it is in my estimation.
My tube amps take about 90 minutes to sound very very good, but I swear that if left on for a few days they enter a new plateau of juicy lush goodness…
I’ll ask Pass Labs and post the answer here.
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From the nice folks at Pass Labs: (the dumbed-down version; me doing the dumbing down)—
My SS XP-20 preamp operates biased to class A. Since this is a very low wattage device, it takes a long time to heat the mosfets to the point where they need to be for their maximum performance and since even small variations in temp will reduce the mosfets performance, a homeostatic state must be achieved and it takes that long to warm not only the mosfets but the environment around them.
The difference between just powered up and 24, or even 36 hours is from unlistenable to sublime.
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Last week, we had a most wonderful vacation in Cartegena and Bogota.
As I left home, I switched off my P20 (my entire system) and the difference in sq between Saturday evening, when I switched it all back on, and now is ginormous. (Just thinking out loud–again)
Very interesting. I had my system shut down for the past 10 days plus as well. Think I’ll turn it all on tonight in anticipation of this upcoming Friday’s listening session…interesting choices for a vacation BTW. My daughter, lives in Cost Rica, has visited both places as well. Said they were great.
We really enjoyed Cartegena. It was blazing hot and humid, but the constant, significant breeze kept us going.
The people were warm and friendly.
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For any particular design, depends in the end on the temperature coefficient (tempco) of the components. I can reasonably surmise it’s likely the transistors that are likely most sensitive. I found my Pass XP-10 and XP-15 both needed plenty of stabilization time. Pass if I’m not mistaken tends to stock and use NOS transistors that it has found best for it’s preamp products and those devices might be particularly temp sensitive.