I know when I redid my setup from a Dennon 3801 to
99/770 I had to tinker with the phase settings on one of the subs to get the base right. It sounded a lot like this problem, it appeared to be moving air, but there wasn't much sonic result. A 180 phase shift corrected the problem. I thought it odd at the time that my Hsu needed no adjustment, but the Velodyne needed phase correction.
I know this was suggested earlier in the thread, but I didn't see any confirmation that it had been tried.