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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.

Albert: Most dynamic speakers will perform worse as they heat up. This is both measurable and audible and may be one of the reasons that drivers that are highly efficient can sound better in some respects i.e. they don't need as much power so they don't heat up as much. On the other hand, E-stat's tend to sound noticeably better after you've powered them up and fed them some juice for a while. Sean
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Regardless of the individual drivers, the caps, wire, inductors and all the interconnect cable benefit from a little music passing through them.

I have owned many cone speaker systems and they always improved with signal. Anywhere from forty five minutes to a couple of hours, depending on the equipment I was using at that time.