Amplifier warm up

A lot of the class D (non pro stuff), says "Leave it on". In the summer anything with valves gets turned off.. 15-30 min in the summer time for a tube preamp to warm up.. An hour in the winter though.. I like 60 degrees in the winter, with valve amps and valve preamps.. It takes about 30 min, if I played them the day before..(winter). The speakers sound better on cool crisp days, too. 

Regards

Many moons ago I bought a Classe solid state preamp.  I was surprised that it had no on and off switch!  I presumed that solid state is better if it is left on all the time!

+1 millercarbon's point about AC noise. OP, if you really want to do a comparison, make sure you do it during the same period -- all during the day vs. all during the evening. If you're comparing evening to daytime, then you're not eliminating the "noise on the line" factor (which may or may not be a factor where you live, but you'd eliminate it by simply controlling the day/time.

My new SMc Audio monoblocks do not have a power switch, just power cords to the wall.  Steve McCormack recommends leaving them turned on for best sound and turning them off for:

• storms and potential power outages
• vacations
• other periods when I do not plan to listen for extended periods of time

Not every amp is going to behave the same and maybe not every amp is thermally suited to remain on all the time....like my former 300wpc Class A Claytons.  I have heard the on/off cycling can be harder on the circuitry than leaving them powered up but.....who knows?

Boz Scaggs always sounds better on a full stomach. My s.s. Ayre amps have a standby mode in which they are always on but not fully powered until ready to use. They fully warm up (to my ears) in about 15 minutes.

Like johnto said, why didn't you just play side A again to compare?