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I'm no expert, but my Sonogy amps have a standby setting where the input stage is powered continuously but the output stage is not. It therefor takes about 30 minutes for them to sound their best as well, as the output stage still has to warm up. Possible that's happening in your case?

In general, Ohm's Law is valid only in 293K temperature and so all of the values of wires, resistors, capacitors and other passive or active elements are specified.

Amplification components run at slightly higher temperatures and as they're supposed to be designed to.
For the best performance the components should rich that best operating temperature.

99% of what's happening after 30min of playing is an additional worm-up of equipment through the signal path.

Please note that very often designer might not test and calculate values of the component for the maximum operating temperatures as I'd say in AVR-s or complexed integrated amplifiers)throughout its whole area and elements location and the benefit you're now experiencing with your components might not apply in that case.

Absolutely! Idle does not keep your system at 100%. Most likely as in my amps, idle doesn't allow current to run to the output transistors thus saving power and heat.

Agree with all the above and add that your source, interconnects and especially your speakers benefit from the warm up period as well.