What exactly is happening that makes warm amplifier sound better than the cold one (when it first started)? This in fact applies to the other components as well; phono-stage, pre, ..etc. but is it equally the same?
Can this change be measured?
thanks
You are not the only one experiencing this. Many folks have noticed this 'effect'
If there is really a change it can be measured.
Some (poorer designed) amplifiers and tube amps require some warm-up time each time when switched on.
Mostly devices that run hot after a while.
One can measure distortion changing (when looking for crossover which is not the same at THD) and frequency response.
Maybe in some tube amps during warm up output R may slightly change.
Some DAC's have chips/Xtals that need to reach an optimum working temp and measurably (not audible) perform slightly 'better.
I the vast majority of cases I would chalk it up to perception. When one listens (while enjoying music) after a while the brain becomes more focussed on music and things in the hearing process 'relax'. The sound is perceived as better. Also the later it gets the better the perception often is.
Now what one can do is leave equipment on and see if the same improvement occurs.
Chances are it doesn't because you know it has been on so are convinced it should already sound optimal.
To check this (over weeks) one would have to really blind test this which is difficult to do so no one will.