Common Ground and XLR?

Does the fact that XLR has a separate signal ground wire render irrelevant whether you have a common mains ground for your components? I've got an ARC LS 16 II and a bunch of components on one end of the room and a pair of ARC D240s on the other. Theoretically both the pre and the amps are fully balanced.

Right now I've got a 36' extension cord going from the amps back to a Power Director near the pre. I'm thinking I might be better off just plugging the amps into the wall near where they are, but I'm not sure that plug shares a ground with the plug everything else is hooked into...

Am I making a big deal over nothing?
High current capability amps like being plugged directly into the wall. The main worry with multiple outlets is whether the hot and common legs are wired uniformly with the same polarity. A handheld outlet tester works great to let you know what's happening there. It also shows whether the ground is present. Ground-wise, in a three prong system all the outlets ought to share a single ground, a rod or pipe buried into the earth. In the old construction, two prong system the common is "provided" back at the power company, and the outlet boxes and conduits may or not be grounded. However, the same polarity issue remains. On old two prong plugs with equal prong widths it's my experience that the ridged side of the lamp cord is common so it goes into the wider or common outlet opening. Orient all the two prong plugs the same way. In any event, dump the extension and have a listen.