What type of wire for dedicated 20A AC lines?

With 10 AWG vs 12 AWG there seems to be strong differences of opinion. But my electrician had no concerns and said the 10 AWG would offer lower resistance and a lower voltage drop over a longer run which I need. That part is totally accurate and I believe is not in dispute.

What IS in dispute is whether said voltage drop with lighter wire is enough to impact performance. But for me, if the electrician is willing to go with 10 AWG, it seems like a no-brainer. There is no downside except a relatively small extra cost.

BTW my electrician said 10 AWG wire is used in commercial applications all the time.

Regarding multiple paths to ground with my two dedicated lines:  I’m not sure yet if I will run my amp off one and my sources with a conditioner off another, or if I run everything on one line with a conditioner.  I guess it depends on how the amp performs plugged directly into its own new line, vs plugged into the conditioner with everything else.  But if I do end up connecting the amp to its own new line, and my conditioner and sources in the other new line, am I heading into ground loop territory?

It’s a known fact that bigger copper wire less resistance easier load .i have awg 10:

in my dedicated  line ,and a 4wire with a common ground ,and key a insulated isolated ground ,with its own buzz bar grid inside the panel and contacts are from Siemens I believe 30 amp Silver plated Copper industrial quality installed several years ago and a big step up in sound much blacker quiet back ground.

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.