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

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.