I just installed a dedicated circuit for my system. I installed 10 awg solid copper. Although 10 awg supports up to 30amp, I used a 20amp breaker. More than I need. I also installed a 240V circuit. I needed it since I bought a European amp that only uses 240V. But I'm glad to have it for any future amps as doubling the voltage cuts the current in half for the same power. This makes your amp feel like it is just sitting next to the power plant with a huge cord. I too highly recommend running your amp at 240volts instead of 120 if possible.
To install 240V circuit the marginal cost increase compared to the 120V circuit is not much. An extra breaker (perhaps $10) and 3 conductor vs 2 conductor wire, and then a slightly more expensive outlet. the main cost of the circuit is all the labor and work inside the wall and cutting wallboard and replacing it. All of that doesn't change when you install the 240V circuit with your 120V circuit.
Doubling the diameter of a wire increases cross sectional area, and approximate power carrying capability, 4 times, not 2.
The power delivered to the speaker is almost irrelevant to the power the amp needs, especially for class A. Not sure why that discussion broke out. My class A 35 WPC amp, that I run at about 1 wpc 99% of the time, uses 400 wpc, either at idle or at max volume.
I reference people back to page one to the post I made with a quote from the AR user manual where Audio Research requires a 20 amp, 12 awg circuit for a 75 wpc class AB tube amp. I know many of you don't believe it can be true unless you hear it from an OEM so it is there in black and white. I copied and pasted it.
Jerry