Best way to extend a dedicated power line?

Use a Hydra 2 in between power cords.

I Use Ilscp PBTS NIMUS connectors.
These are what 90% of the electricians use when splicing wires together. They use screw terminals to make the splice secure. Make great mechanical connection and will not become loose because of heat causing the wire to expand and contract. They are totally insolated and will pass any electrical code requirement.
You can get them at Home Depot.
The PBTS-2-4 are good for wire from 4 to 14 gauge.

Crimp type butt splices work great on stranded wire but not on solid. Only if the wire connection, after making the crimp connection, is soldered will the butt splices hold up with time. If not soldered, in time the splice will loosen causing arcing in the connection. Arcing causes heat....
Arcing also causes AC noise.

I would recommend splices be made up with a spring wire connector like the yellow or red Ideal brand.

Scotchlok brand work great as well.
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Jea48, yes, I used a similar product. Haven't had any luck finding audiophile grade butt connectors.

11-17-08: Sns
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

? Crimp in line butt connectors?
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Those are good data points. Thanks both!

Horacio

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.

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.