Edwyun 2-22-2017+1. Specifically, it’s possible that there is nothing "wrong" with the AC wiring to the listening room, but it may have different characteristics than the more recent wiring to location 1, which in turn are bringing out an inherent susceptibility of the power amp/preamp combo to ground loop issues.
There can be a combination or cumulative effect of issues.
More specifically, see pages 31 to 35 of this paper, written by a renowned expert on such matters, in which he describes "what drives 99% of all ground loops." As you’ll see, Romex (which perhaps was used in the relatively recent wiring of location 1) is much less likely to bring out the susceptibility to ground loop issues that is inherent in some designs than wires randomly positioned in conduit (which perhaps was used in the older wiring to the listening room).
If that is what is occurring, and it seems like a reasonable bet at this point, a solution may be to have an electrician install new wiring to the listening room, using Romex. Alternatively, inserting a suitably chosen Jensen transformer between the preamp and each of the amps would probably resolve the problem, while also perhaps reducing other forms of noise, at a cost of around $300 plus some additional cabling.
Reports that have been provided here regarding the transparency of Jensen transformers have generally been very positive, although I can recall a couple of people who have provided comments that are a little bit mixed.
A suitable Jensen model would be the PI-RR, which would accept an RCA cable at its input and provide an RCA output, or the PI-XR, which would accept an XLR cable at its input and provide an RCA output. You would need two of them, one connected via a short RCA cable to the input of each amp. The cable from the preamp to the Jensen transformer could be whatever length is necessary. Here is a good supplier, although they can also be ordered directly from Jensen.
Also, although it is apparently unrelated to the problem I would still strongly advise that at some point soon you eliminate the XLR-to-RCA adapters, that are shorting the signal on pin 3 of the preamp’s XLR outputs to ground. If the sub does not provide XLR inputs you could either special order adapters from Cardas which leave pin 3 open, or have a cable supplier custom make an XLR to RCA cable which leaves pin 3 open, or purchase a Jensen PI2-XR transformer which would provide two channels of balanced to unbalanced conversion. Or you could use the preamp’s XLR outputs for the main signal path, connecting them to the amps via the Jensen PI-XR transformers, and connect RCA cables from the preamp to the sub.
Good luck. Regards,
-- Al