I've definitely noticed that SS sounds better when left on all the time. I only have a class a/b integrated and phono stage that I leave on, so it's not using much power when not in use. Recently, I went out of town for a few days, so I powered everything down. When I returned, I was looking forward to hearing my rig. Those first few albums were pretty painful - everything sounded thin and glassy. The next night, things were sounding great again. I really wasn't expecting such a difference, but it wasn't subtle. That being said, if I had a class A amp, I wouldn't leave it due to environmental concerns.
Amplifier warm up
I was plalying Boz Scaggs, "Silk Degrees" LP. After side one, I ate dinner. One hour later I played side 2. The second side sounded clearer, more vibrant with better sound stage. I have a Parasound P6 preamp and Parasound Halo A21+ amp. Is it possible that an hour of warm up improved performance?