Internal Wire Gauge?

2.0 welding cable should be fine. Make sure it's grounded well.
Pure silver multi strand, I figure about 25k for cable, 500.00 valve amp..
LOL

Beat me to it! :-)

The wiring should be UL1015 listed

That's really specific and unnecessary. UL1015 is 600V  PVC shielded appliance wiring material. It's in an enclosure, there is often no need for shielding at all, and it would be detrimental to the build in many locations.  Where shielding is required, there are any number of UL style wires that would be suitable depending on where used, how supported, etc.

And yet $100,000 CD players (and amps) are built on PCBs using generic copper, typically FR-4 substrates, though I have seen the odd PTFE, though their is little justification for it.

PTFE (Teflon like) insulated wire does not need to cost an arm and a leg, it is common for industrial and military uses and costs a fraction of "audio" wire..

Then again, using PTFE for hookup wire is pretty silly unless you are running twisted pairs (or similar).  Individual wire runs don't benefit for a low dielectric constant. You need two conductors to have a capacitor, so if one wire is 10mm away from another wire, and you have 0.1mm Teflon insulation, the air is still by far the dominant dielectric.

If you are selling things and want to impress people who don't know any better, sure use expensive wire that will make no difference. However, if you are building your own stuff, every dollar you don't waste on wire can be used in places where it will actually make a difference.

Yes bad use of wording by me, jacketed is more appropriate, as opposed to shielded, however, there are a ton of wires that would be suitable, meet UL (or other standards), and have a 600V rating. However, unless you passing near another wire and/or the case, you often don’t require jacketing at all, bare wire may be suitable (just like a PCB), as long as the wire is supported. The op is asking about internal wiring, which could be many spots. I normally recommend for those not great at soldering to invest in a stripper that is good with teflon wiring and then use teflon wiring as excessive heat not not significantly damage the jacket.

1015 is often used as it is very cheap and provides better protection and colder guaranteed temperature rating than 1569 for 277V circuits.