Amplifier warm up

Yes.

Temperature changes will change the biases in transistors and hence make differences.

It is almost like an ICE engine.  It runs better when it is warm.

Unless your amps are choked for air in a cabinet I've never been a fan lol .class A gets so hot its hard to even touch and I would leave on 24/7 . My hybrid tubes get hot!
imho

I leave mine on 24/7 unless there is a serious threat like a storm.

There are a few other possibilities that were not considered:

1. Side One of the LP may be your favorite and thus it may have gotten more play and is showing its age compared to Side 2 that is more pristine.
2. By the time you had finished your meal, the power grid in your area may have become less busy and therefore better suited to your listening session.
3. That bong hit.

atmasphere8,812 posts10-27-2020 3:47pmYou can't take any good system seriously for the first 1/2 hour or so!

Indeed - also your phonostage and cartridge need warming up. With vinyl things can be very frustrating. Part of that early dissatisfaction will be augmented by the phonostage and cartridge not performing yet.