20 Amp Dedicated line: 10 gauge vs 12 gauge wire

Keep in mind, though, that if the interconnects between components that are connected to separate dedicated lines are single-ended (unbalanced), there is the risk that voltage differentials between the grounds of those separate dedicated lines will couple noise or hum into the system via the shields of those interconnects:

http://www.jensen-transformers.com/an/an004.pdf

Regards,
-- Al

A good practice in installing the dedicated circuits is to keep all the runs of the same length to deal with the potential Almarg mentions.
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Even with the same length for all runs, the voltage drops will be different for each circuit. This is because power amp draws more current and it would see a slightly lower voltage.

There is no issue running 10-gauge wire for a 20-amp circuit. I have a circuit run with hand-pulled solid-core 10-gauge and high-grade conduit that is 100' from breakout box to listening room with great results. I use Oyaide R1 cryo'ed outlets and have found like others on the thread that removing my power conditioner after the fact (Furman IT-Ref20i) actually resulted in even better sound. The comments on ground potential over multiple dedicated circuits as written above are also well-founded. One suggestion I would make is to move away from Romex; there are better sounding alternatives out there in terms of both stranded and solid core options that will cost you more but be more than worth it in the long run (pun intended)!

By all means, I'd be interested in alternatives to Romex.