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.
I will take a pass. If you really want to know, it has all been answered on my system page,    and in even greater detail in my posts. Search around it is all there.
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.
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.
I just did a search & came up with this; While it’s common to use a 6-gauge wire for 60-amp breakers in practice, it’s best to use a 4-gauge wire if you’re installing a 60-amp subpanel. 60-amp breaker panels controlling several circuits can draw a max of 60 amps before the subpanel breaker trips. I doubt you’ll ever draw 60 amps @ any giving time.
It doesn't matter what anyone doubts what matters is passing inspection. Call an electrician.
I have an electrician on site working on our house already. It is a new construction build, but maybe 90% complete.

So, a single 4-gauge from a 60-amp main to a subpanel. And from subpanel use 10 awg to outlets? Just to be clear, this method is preferred to using 4 separate cables (e.g. 8awg or 6awg) from main panel and using a junction box (instead of a subpanel) just before outlets?

Can you get specific on exactly what cables? Any specific brands and/or characteristics? i.e. Stranded vs solid core? Is there a solid core option for 4 gauge?

My electrician probably knows, but also I don’t want to assume anything, since this is hifi related and maybe he would choose something suboptimal.
I don’t think the 4 gauged wire matters, but I suggest VH audio’s 10 gauge Cyro treated wire. Try to look into an audio grade sub panel or something of a Copper buzz bar sub panel.
Talk to your electrician. He can place the sub-panel, you could probably handle the rest from there. Don’t attempt to saddle him with a bunch of unusual requirements that you really have no knowledge of. Tell him you want quality materials, panel with copper buss work, copper wire. Give him the basic idea of what you’re trying to do, and why. If he’s onboard, let him do his job. That’s my advice, anyway. I’ve worked in residential construction for 32 years. Good luck.
The wire won’t be a single, but individual insulated #4 wires, THHN or THWN, I believe, in PVC conduit from panel to panel, and might also include a bare ground. Not sure about that last bit, I can’t remember the code requirements for sub-panels. I’m not an electrician.
I just reread your post about the ongoing new construction. Of course, I don't know where you are, but here, our electrical inspector would more than frown on you doing any electrical work while your electrician is doing the job under his permit. Ask him.
If it's underground it would be the THWN. W for wet. Most  #4 is stranded aluminum .
Far better to run a single 50/60 Amp 220VAC line to a sub panel.  You'll have lower voltage drops.  You can then put a balanced 220/120 VAC transformer there.  Noise, voltage, all optimized.

djones514,201 posts09-14-2021 6:58pmIf it's underground it would be the THWN. W for wet. Most #4 is stranded aluminum
Most is, solely due to cost. Copper is probably 5x the price.
Nothing like a pass and a pick six.
Hire an electrical pro and forget about it.
First thing you should do is figure the current draw for your equipment. 
A little overkill is ok but why get carried away?

In a previous job I rewired our ISP buildings with #10 stranded for all 110 VAC circuits, always with a separate neutral (no sharing with other circuits). This was in case we ever needed to change an equipment rack from 110 to 220 (or possibly 208). And yes, stranded handles more current and dissipates heat better.  

Also a common mistake in sub-panels, whether 110 or 220 is bonding the neutral to ground. The is the rule in a main panel but a no-no for sub-panels. 
Where I live it’s illegal to run the four separate lines, so I’d run a 220 line to a sub panel and then run my separate lines from there. It’s also much easier and safer that way.

All the best

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? 
''Does more voltage drop also hurt sound quality?''
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.
As others have mentioned, talk to your electrician.  You don’t need any exotic outlets or special parts for this.  Just ask your electrician to use better than contractor grade outlets.  Make sure you have individual grounds run to each outlet. The ground wires should not be daisy chained.  
If you will be drawing anything near rated capacity on all four circuits you may want to be thinking how you will be keeping your room cool.  If each circuit is loaded with 10 amps of equipment you will be drawing and essentially creating 4800 watts of heat. Which converts to about 16,380 BTU/hour or about 1.36 tons of cooling.

You may want to add up the maximum  current draw of all the equipment you will be using and see what the total is.  Don’t expect to be actually drawing that many amps/watts on a regular basis, only under extreme use conditions will you ever be hitting the max on all equipment.

if all your equipment is placed in a rack or closet you want to be moving a significant amount of air through that space to keep the equipment from overheating.
Forgot to mention:

If you want to go down the rabbit hole that is researching 20 amp receptacles do a search on either Hubbell 5-20R or Bryant 5-20R.  There will be literally hundreds of options.  Check out commercial or industrial grade.  Or for the ultimate check out the hospital grade receptacles.

Secret  tip: many or most of the “audiophile” versions are just rebranded items from the regular electrical parts manufacturers with a commensurate price markup.
Use a subpanel absolutely. Use largest gauge wire to minimize wire resistance and voltage drop for the best results. +1 gs5556. Your weakest link will be the connectors, silver plated high grade outlets.
Awg 10 would be idea Copper, no aluminum wire 
awg8 even better but even high quality Gold copper outlets 
awg 10 is roughly near max .
Okay all, spoke with my electrician. We will be using #1 gauge from the main to a subpanel (I think 150amp subpanel?). And from the subpanel will be using 4x VH Audio 10-2 Cryogenically Treated Romex to outlets.

The electrician also brought up the idea installing a power conditioner at the subpanel. But I wasn’t sure how this would interact with my ability to use a high end power conditioner (e.g. niagara 7000) with my system. Any thoughts on this one? The electrician didn’t have a good answer for me on how an installed power conditioner may affect my system’s power conditioner.

As of right now, I told him to not install a dedicated power conditioner at the subpanel, but still have some time to think about it.

Thanks for everyone's input so far, very helpful!
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?
My experience with rhodium plating is it is very pretty but a terrible interface to another surface.

It is too hard to allow for a great grip. Copper alloys, like on a normal wall socket or spade, are soft and deform readily. This malleability makes them deform and make a more complete and tighter connection to another metal. This is something you can readily feel when making a hard to hard surface connection, like nickel to nickel or rhodium to rhodium. It never feels as tight and secure than compared to using a soft to hard or soft to soft connection.

For instance, a nickel plated speaker connector has a very hard time staying tight onto a hard spade. Switch the spade to say gold over copper or the WBT spring loaded spades and the problem goes completely away. I have also used rhodium AC connectors and faced the same problem. They just do not grip or stay in the outlet as well.

The worst high end connector I ever used was a Furutech locking banana. Maybe I got a fake one, but it was just like half a millimeter too small, and the rhodium plating was TOO smooth. It was nearly impossible to make a strong speaker connection. The inexpensive solid copper spades were far superior and a lot less expensive.

Not saying you should avoid Furutech, but all of my experience in various locations (AC/speakers, etc) says rhodium is not very good except to look at.
I see, interesting. Do you have another receptacle brand you can recommend? 
I’m not saying you should avoid Furutech, but avoid Rhodium plating.

Oyaide and Leviton are fine brands as well.

Here’s a great source:

Chris is a great guy, I’d literally call them for advice. If you stick with Furutech, I'm going to recommend the gold over copper as a much better performing outlet:
This article about how to wire your house for good power will tell you everything you need to know. It’s excellent.