Should Subwoofer Be Plugged Into Same Circuit?

To add to the foregoing comments, I suggest that you read pages 31 through 35 of the following document, the section that begins with "this finally explains what drives 99% of all ground loops." It was written, btw, by Bill Whitlock, a renowned expert on such matters.

https://centralindianaaes.files.wordpress.com/2012/09/indy-aes-2012-seminar-w-notes-v1-0.pdf

As you’ll see, the type of wiring can dramatically affect susceptibility to ground loop issues. Romex (i.e., NM cable) is pretty good in that respect. The worst case in that respect is discrete wires randomly positioned in conduit.

My thanks to member @Jea48, this Forum’s foremost expert when it comes to electrician-type matters, for calling this paper to our attention here a few years ago.

Best regards,
-- Al

I’m glad you guys jumped into this convo because I have another question. 
Notice I said same circuit.

i need to branch an outlet off to reach the sub in the corner of the room. 
I don’t see why the sub would have to be plugged into the exact same outlet vice a branch outlet in the corner where there is only the two outlets in the circuit.

please advise.

My guess is that you'll be ok plugging the sub into a different outlet than the rest of the system, on the same circuit. Assuming (per the reference cited in my previous post) that the wiring between outlets is Romex or some other wire type which has the safety ground conductor symmetrically placed between the two current carrying conductors, or alternatively has them all twisted together.

The only way to know for sure, though, is to see how it works out, as susceptibility to ground loop issues is also very much a function of the design of the specific components.

Best regards,
-- Al

Thanks Al! So why is a 20 amp circuit recommended over a 15 amp out of curiosity? 

In addition, @almarg correct me if I'm wrong, one of the receptacles needs to be grounded to the service panel, not self grounded. If there are two separate runs of Romex, then both need to be grounded to the panel to help prevent a  ground-loop. This may also lower the noise floor.