The M60 appears more linear to me than the M5 so that's a good indication it ought to disappear as well. Then again, the published curves are for the standard M60. I have no idea what the M60HP curves look like. All it takes is some cabinet vibration of the right amplitude and bandwidth and BLAM! the speakers become visible.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated