Best way to extend a dedicated power line?

It seems you have two choices. One is to add a junction box, either before or after your current dedicated outlet. The other is to just use your current dedicated outlet as the junction and run the wire to the new outlet from there. It would seem that as a practical matter, it might be better to use the latter approach. That way, if your equipment location changes in the future, you still have the original outlet in place. If you're not using the older outlet, it's still a dedicated circuit. I don't think it's really an issue in terms of sound quality unless the wire is picking up interference or unless the wire is very, very long.

I recently went through the very same thing. I just butt connected the new romex to the existing romex. I tried to find some 'audiophile' butt connectors, but no luck so I ended up using a commercial butt connector I coated with the Quicksilver Gold contact enhancer. I also used the cryo'd romex VH Audio carries. I expected possible sonic losses, just the opposite, I thought I heard incremental gains in refinement, resolution.

Those are good data points. Thanks both!

Horacio