How to A/B Test Power Cables & Interconnects?

Two homeruns are identical length romex, and on same phase. Seperate junction boxes. Grounds go to same panel, but not sure about them being "tied together." My thought was one circuit for power amp and the other circuit for all other components. 

XLR from DAC and Phonostage into PreAmp, and XLR from Pre to Power Amp.

I have not tested it for noise yet,....

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.