If you didn't try swapping wires at the receiver, I'm still confused about how you tested for phase coherence (polarity consistency) among the front three speakers. Did you use a test disk or is there a Audyssey routine within the receiver or something?

And, yeah, just try a sub in there (in various positions) and see how that might work. That seems like less trouble than moving the M60's, and I think it probably gets you closer to your ultimate solution.
bibere usque ad hilaritatem