Dedicated 20a lines, 125 feet distance, which gauge wire to run?


Want to set up 4x dedicated 20-amp lines for my hifi system. The distance from the outlets to the breaker box is about 125 feet. What wires should I use? 10, 8, or 6 gauge? And should they be solid or stranded? If you could be as specific as possible that would be appreciated (brands, links, etc), as I am out of my depth here. The wire will need to run underground for a good portion, and then into craw space, if that makes any difference.
Ag insider logo xs@2xbattbot
125 feet is quite a long run and you should consider using a subpanel. Even with #8 wires you will lose 3 volts at 15-amp draw. A subpanel with minimum #4 wire will limit voltage drop to a little over one volt at total current draw of 15 amps. From there go with 10/3 romex to the outlets. The 10/3 comes pretwisted and that helps eliminate common mode noise. Never run 10/2 parallel romex, that acts like an RFI antenna.
Does more voltage drop also hurt sound quality? Or does it just affect how much total power my system can draw at a given time? 
No, voltage drop isn't a worry unless it exceeds 5% from the transformer to your outlet then it's not code. Quit obsessing over irrelevant stuff and listen to your electrician. Get him or her to test the voltage on an existing outlet and see what it is. I've had 121 volts on  a 120 volt outlet. It can vary during a 24 hour period. 
Call an electrician and install what code says. Anything here that is spot on with your local requirements is merely coincidence.