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.
I was a Calibration Specialist in the Army (man that was a long time ago...), using reference level equipment to calibrate lesser-level test equipment (O'scopes, Distortion Analyzers, Volt Meters, etc.). I'm sure the reference level gear we used was pretty darn expensive as was the level above ours that was used to calibrate our gear.
The first step in most calibration procedures was "Turn the unit on and wait 30 minutes."
