Unplated copper spades/banana

Why do we need to have high skills to listen or enjoyed music?

twisting it to the Romex and using two wire nuts (not recommended and not approved by code!)

Hardwiring is code-compliant, you just need to provide an appropriate junction box. 

Using "two wire nuts", however, would be a code violation as it would leave one of the 3 conductors unconnected :)

Hardwiring individual components may not be ideally convenient, but it would seem appropriate for a power conditioning unit.

If you can find pure silver wire with pure silver connectors I would assume this would be the best conductivity of signal one could get.

https://tempoelectric.com/cables.htm

Copper is more conductive than gold.  If you're worried about tarnishing, put a thin layer of silicone grease on the bare copper surface.  But will you hear a difference?  Does it matter that you have a pure copper spade with no plating if the jack is nickel plated?  Let's assume even the jack is bare copper, will it matter while the leads of any resistors or capacitors in the signal path are a tinned metal inferior to bare copper?  Will it matter if you have pure silver speaker wire if the interconnects leading to the inputs of your amp are copper running through plated brass jacks, which are fed by copper traces on a printed circuit board?  Honestly, where do you draw the line if you're going to chase nonsensical things like this?

There are bigger fish to try, such as upgrading to better quality electronic components within your current equipment.

In theory sure bare copper or silver is more conductive than gold plated connectors. In use, there’s no way anybody could hear the difference all else being equal. We’d be talking about a fraction of a decibel that’s just not close to being audible. So it makes sense to use gold over copper to avoid corrosion.