warming up the system

It takes less than 30 minutes to break in a new vaccum tube.

There is no dielectric.

This has been discussed at length in a couple other threads.

Warm up for tube amplifiers averages 20-60 minutes.

As to a solid state amp if you have measurable or auditory changes in the performance of the amp after it has been on for 2 seconds, then you paid too much for it, whatever you paid. A design that is that unstable with temperature is not worth the paper the schematic is printed on. As to tube amps, perhaps a little longer due to the heaters. What does happen is the listener gets used to the sound, the sound doesn't change. Same thing for speaker break in past a short period of initial use - less than an hour. Cable break in is ridiculous - but a good rule of thumb is just past the time for the warranty to expire.

My old Rowland 7M monoblock furnaces take about 8 hours of playing time to stabilize sonically and yield their best when starting from standby. . . . more if you turn them on from totally off-cold. Current cool-running JRDG 312 takes longer than 7M. . . couple hrs playing after idling in ON position overnight; a day when bringing online from idle; couple of days when starting it from long off-cold. . . . admittedly, I am anally retentive. . . or delusional as some may suggest.

According to Pass Labs they recomend at least 1 to 1 1/2 hours for there amps to get up and running.

"but a good rule of thumb is just past the time for the warranty to expire."  Classic!