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.
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I use 10awg on 75' lines. The problem with anything bigger is you'll have issues with fitting onto vast majority of AC receptacles. You'd probably have to use sub panel for the larger wire, then run 10awg from there.
It’s no problem for me to use a junction box for say, 8awg, then switch to 10awg from there, as there is crawl space right underneath where the dedicated 20a outlets are going. I am going to use floor outlets. 
I would put a sub panel in and then run your 4 dedicated outlets from the sub panel.
Do you have room to put a 60 breaker in your main panel? If so ,run 6 gauge wire to a 60 amp sub panel, then run your receptacles from there.