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

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.