Four Hour Tube Amp Warm-Up

@mulveling "However, it is very difficult to separate any such sonic differences from psychoacoustic explanations"

This is the Achilles' heel for many discussions. Obviously, the OP knows his own mind best, and we have to stipulate to what he's hearing. That said, 4 hours is a very long time, certainly an outlier in terms of what others report and what the equipment should be able to do. (Imagine a car that took a full 60 seconds to reach 60 mph --- that would obviously indicate a problem or bad design.)

As for your comment about the fallibility of SS -- good point. Tangentially, I'll confess that my Pass XA25 takes about as long as my KT88 tube amp to sound decent, and it sounds great after it's been on a long, long time. 

bigtwin wrote: "I'm not aware any any manufacturer that suggests leaving equipment powered up 24/7?  I would think you are only shortening the eventual life span of things doing this?  Just asking."

When I bought my Pass Labs XP17 phonostage I called Pass and asked if it would be OK to leave it on 24/7.  I was told that it was designed to be left on all the time and that is why the only off/on switch is located on the back of the component. 

From the Pass XA25 owner manual...

Leave it on or turn it off: The product is designed to be left on all the time. The exception to this might be a case where the unit will not be used for an extended time - in that case it can be turn off. The main power switch is located next to the AC power cord. (THIS REFERS TO THE REAR MAIN POWER SWITCH)

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You are now ready to play music. Turn the level at the source or preamp to a low level. Press the power switch on the front panel of the XA25, the blue LED above the power switch will light. Turn the volume up to your preferred listening level. Sit back and enjoy! The XA25 takes approximately an hour to fully warm up and sounds better as it does. Pushing the power button on the front panel of the XA25 a second time turns it off and extinguishes the blue LED. (THIS REFERS TO THE FRONT PANEL POWER ON PUSH BUTTON)

@bigtwin Heed Audio, Simaudio, Pass Labs, SPL, AVA, just to name a few. 

bigtwin wrote: "I'm not aware any any manufacturer that suggests leaving equipment powered up 24/7?  I would think you are only shortening the eventual life span of things doing this?  Just asking."