Is my Pass amp overheating?

The amp is an XA30.5.

Yesterday I was doing some tinkering and the amp was turned off for a couple hours (unusual for me). When I was done, I sat down to listen while the amp was still cold. To my surprise, it sounded BETTER than I'd ever heard it.

The amp is Class A, so according to conventional wisdom, it needs to warm up before sounding its best. Yet it definitely sounded better BEFORE it warmed up. So I'm wondering if the amp is overheating. To test this, I took the lid off the amp. Three things happened:

1. At idle, the bias needle moved from 11 o'clock to 1 o'clock (unprecedented).

2. The cooling fins became much HOTTER (not cooler, as I would have expected).

3. The sound quality of the amp remained "improved" even after several hours.

Can someone please explain what's going on?

Thanks,
Bryon

P.S. The amp is in a closet. But I don't think that's the problem, for the following reasons: The amp is on the top shelf of an equipment rack, so there is two feet of open air above it. There is a large fan in the ceiling of the closet that sucks air and sends it through a duct to the outside of the house. So the closet stays very close to the temperature of the rest of the house. Also, I can reproduce all the effects described above with the closet door open.