I had them flanking the center channel for a few weeks, and there was some midbass cancellation going on there. I guess my next move would be to place them close to the MLP, at each end of the sofa.
I have measured a crude response that is more or less flat and slightly rising toward 20hz, from 20-80hz with the subs stacked at the front. The subs are summing at +4-5db in the deep bass, and filling in a deep suckout (14db) at 50-60hz