Hello,
I been an audiophile for over 20 years now. I have owned about 20 amps and have had long in home audtions of maybe 20 more amps. I can tell you without a doubt that everytime and with everyone of them, from tube to solid state amps, there is a warm up time!
The sound changes mostly for the better and on a very select few amps (3 of them that I can remember) after warm up the sound changed and became too bright for me and not to my liking.
Anyways, for the most part I hear the difference from 5 minutes in to 30 minutes in to an hour in and even sometimes 2 hours in with some amps.
There is also a break in time if the amp was purchased new that usually takes 100-200 hours to break in.
The real character of an amplifier reveals itself after warm up and break in time. This applies to CD payers and Record players as well.
Anyone who tells you differently needs to go and get their hearing checked.
Warm up time is well established in the Audiophile community and seasoned reviewers bring this subject up often when reviewing equipment.
A car engine runs better and smoother when warmed up.
A person also plays better sport after being warmed up.
Tell your wife an oven cooks better food when it is warmed up
Cheers :cheers: