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.