How to A/B Test Power Cables & Interconnects?

If you have separate junction boxes for the receptacles, then I would assume the grounds are different (at the receptacle). 

All equipment has either intentional capacitive connections and/or parasitic connections from internal circuitry to case-ground. Without your equipment connected, except for the AC, those grounds connect back at your electrical box as you have it wired. As they operate, they inject signal into the ground, which is now separated by two long wires which means that ground on one set of equipment is going to be different from ground on another set of equipment. That ground difference is injected into the connections between your equipment.

As you are using balanced connections, fortunately most of that noise is going to be eliminated, but keep in mind, that noise tends to "pump" with the signal, so your floor with no signal may be really quiet, but that does not mean there are no noise issues.

If all the equipment has the same local ground, then the ground noise is only injected over the lengths of the power cords, not the length of the home-runs (2x) plus the power cords. As the injected current will be the same in both cases, the longer resistance of the added 2x home runs will make for a bigger noise signal voltage.

OK. How do I address this? What is involved in getting both circuits on a common ground? (Assuming that is the solution.)

That part is easy. Just bring your two home runs into the same junction box connecting the ground from each to the J-box metal, and then to each receptacle. 

If you have a modern breaker panel in the U.S., all of your grounds are joined at a single bus bar in the panel, and then go to ground. If you have dedicated circuit(s) going directly to the panel, you shouldn't have any metal electric boxes for outlets. Or anything else. They're obsolete, and a liability in a modern electrical layout, in all but a few exceptions.

By code, they (grounds) have to be all connected at the panel to a single bus-bar do they not?

Perhaps obsolete, and I wonder if regional, but I am still seeing more metal boxes even in new construction, in north east U.S. and Canada, but pretty much all plastic with integrated vapor barrier for the exterior. Are we talking the same thing? i.e. ... https://www.homedepot.com/p/2-Gang-Drawn-Device-Switch-Box-with-Bracket-685SP/100154820

Sounds like you know this well. I would be interested in your insights ... and how it relates to mechanical as per the next paragraph.

While metal boxes have higher potential for "liability", if they are improperly grounded and a live wire touches the box, and a person touched a screw that attached to the outlet cover that attached to the box, then yes they could get a shock. (Or you cut the romex on the sharp metal). On the other hand, with people hanging literally pounds of audiophile AC cables off them, and multiples at that, the significantly higher rigidity of metal could balance out any benefits of plastic.

That said, for the DIYer, plastic can be safer (and easier), and since the gauge used was 10, it should be easy to just run each ground to the separate receptacles (in the same j-box), and then run a ground wire between each receptacle so the grounds are at the same potential.