Amplifier warm up

+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?

It's certainly been my experience that noisy AC lines are a culprit for variability in sonics along with warm up time.  With all my gear over the years 1 hr seems to do it for warm up benefits...more just doesn't help much for my ears.  But the time of day has huge sonic swings and compelled me to eventually get a PS Audio Power Regenerator.  It even measures the distortion coming into the unit and I can see it go from more than 4% THD during peak area usage periods down to 2% late at night or very early in the morning.  With the regenerator the output is consistently below 0.1% THD which has seriously enhanced my listening experience.  Plus it regulates the voltage to a consistent 120V....otherwise mine fluctuates 118-123V otherwise throughout the day.

Guys,
You need to get off the grid....check this out....better than $10K PS Audio:

http://enjoythemusic.com/magazine/equipment/0119/Goal_Zero_Yeti_400_Lithium_Portable_Power_Station_R...

You really don't need a Goal Zero with its fan noise.  You can buy low distortion sine wave inverters for a few hundred $ and a 100 amp hour battery or two and a big charger and run your system all day before needing to charge.  You can listen while charging but it won't sound as good.

As good as inverters are (completely off the grid and better sound than practically any line condiioner) they still make noise.  My friend is using the Puritan line filter after his 3000 watt inverter and it sounds even better and it isolates each component from each other.  Incredible sound....same sound day and night.