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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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.
Since you’re out of your depth I hope you get a licensed electrician. You will need direct burial or UF wire underground. If you put in a 60 amp sub panel use 4 awg from main to sub then at least 12 awg to the outlets. 6awg is only rated for 55 amp you need at least 4 awg for a 60 amp sub panel. Talk to an electrician.