This is legit comedy gold...
A gentleman I work with holds the Bell Labs patent on high speed data lines with negative phase cancelation. AKA, twisted pair ethernet wire. You’re at a low enough frequency that the advice given on phase cancelation doesn’t matter. It isn’t relevant. I did grounding and bonding in telecom for years, none of what you're saying above will work. It will not cause blacker blacks, deeper blues, or crisper highs.
Spend the money on a new record or cartridge.
Your recommendations to twist and cross wires won’t help significantly if the lines are linked back on a common buss bar in the panel box, or share a common ground (they do.) To truly isolate your signal, you'd need a filter bank off the pole (we used massive banks of lead acid batteries), float your ground, and re-establish it elsewhere. You're not doing that in residential construction.
8AWG will give you no more current than 12AWG of the amp doesn’t need it. It doesn’t if it’s not 240v. Nothing sold 120v in the US exceeds 15A draw. For a reason. It doesn’t meet the National Electric Code.
If it’s continuous duty above 15a, they make the device 240v. Small surges (transients or power up Cap "filling") are fine on 12/2, and for big amps, they should have a timed/gradual circuit to handle in-rush current.
Go to Lowes. Buy 12/2 romex. Buy a decent plug (Leviton premium, hospital grade if you want it to grip harder), be done.
If you have noise in your circuit, fix the ground on the offensive item (usually a motor.)
If you still have noise, buy an amp with a better power supply design. Only time I ever had noise existing on an actual AC circuit, from the pole, was at a friend’s house near a 60kW AM station’s antenna array. I doubt you’re in the same boat.
