Question For My Electrical Expert Friends

As long as you are not using the internal straps on the receptical in the current recepticle to add on the romex.
You need a good solid connectiond of the added portion to the already installed portion.
As long as the connection is solid in there and not some flimsy bit, it should be good.
So making the splice the important bit, and not the connection to the original duplex outlet the priority. Inside that first outlet.

I would make a true equal splice in there, with only a pigtail off for that original outlet. So the current can really flow to the new added wire.

Thanks _E...

I plan to remove the original outlet and make the "splice" Romex to Romex with some wire nuts. I imagine this should not negate the intended purpose of a true dedicated line.

Ensure you ground properly with the pigtail.

Are you using audio grade receptacles or upgraded receptacles that are more sturdier?

Yeah. With the splice being done with wire nuts directly from one Romex to the other, it should be all good.
Make certain the nuts are TIGHT!
And, remember copper will soften over time, so go back a few weeks later and retighten those connections.

I'm not sure there is much of a difference electrically between running a splice with wire nuts and plugging in an additional 8 ft of extension cord, ASSUMING the extension cord is the same or higher gauge than the Romex. With the extension cord you have a choice of using much better multi-stranded wire than the solid wire in romex. Face it, Romex is not high end wiring. Also, if you are using hospital grade plugs and receptacles (or even better audio grade components) I would expect the extension to be better. The extension may not be cosmetically pleasing, which is another issue.

Plus, if the extension is plugged directly into the power amp with an IEC plug, you save the additional interconnect of the splice and the interface contact with the second receptacle.