Dedicated power

You don’t want the AC line mains voltage bouncing around in beat with the music when pushing the amp hard. An amplifier’s power supply like/needs the AC line voltage steady state.

The power line does not bounce around in tempo with the music. The filter caps supply DC current modulated by the output devices. The line charges the caps @ A/C line frequency. see ieLogical CableSnakeOil and http://192.168.1.160/assets/CblSnkOil/Signal_vs_ACLine.png and Linear Power Supply Design (sound-au.com)

For the umpty-billionth time, PS capacitors charge in sync with the line frequency, NOT THE MUSIC!!! A 15amp circuit with clean, screwed terminals can supply all the current necessary to vaporize any voice coil. The only time the full line current is used continuously is when testing on a load resistor.

If the sound collapses, the PSU is inadequate to supply the required current. Either the caps have high ESR or their capacity is too low.

@OP - If you plan to purchase dinosaur amps, plan to recap them. Otherwise your fat wire install will be for naught. Capacitor technology has advanced considerably in the last three decades.

residential electric service is 240VAC, which is achieved using two 120VAC lines that are 180 degrees out of phase with each other.

They are 180 degrees difference when measured to neutral not each other.

People may balk at this, but it is my experience.  I ran two 20amp dedicated lines from the first breakers in the panel, everything else on a different leg, 10 ga. Solid core, in conduit, straight run, to the outlet box. Also placed a separate grounding rod for these two lines. While the improvements were vast, noise from all the household appliances, etc. Were still present. The solution was to use isolated ground hosp. Grade outlets sharing the grounding rod. Cleanest, strain free, quietest power I have ever experienced. Art Audio High Current Jota mono blocks on one line, original Hydra on the other feeding preamp, digital sources & analog sources.  I have shared this on a few boards over the years, but never known anyone to take this suggestion.

FYI, You may not use a ground that is electrically separate from your service entrance ground.  Doing so violates the NEC and is a safety hazard.

You may add additional ground rods to the original circuit however.