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 assume the power conditioner at the subpanel would work on all 4 lines. What are you intending to plug into the power conditioner in the system room? If you plug the e.g. Niagara 7000 into one outlet then components into the Niagra what’s the point of 4 lines? Or are you going to put a power conditioner at the end of each run  for a total of 4 ? Am I missing something?
#1 wire is a lil over kill but, go with it. Do you really need a 150 amp panel? Just make sure the buss is Copper! I'm happy to see your going to use VH wire. Looks like your on your way. Keep us in the loop.
The power conditioner at the subpanel would work on all 4 lines, yes. I was planning to have the Niagara 7000 on it’s own dedicated line, with all electronics connected to it. And 2x MC901 monoblocks each on their own dedicated line. So, 3 of 4 dedicated lines allocated.
For me, no Niagara at the subpanel. That's just a bad idea if you have a problem, you'll need an electrician again unless you're handy. Good ideas on the wire, your electrician sounds competent. Silver plated outlets, not gold.  
For the receptacles, I was planning on using 4x Furutech GTX-D NCF(R) AC Receptacles. Is there anything better?