"The woofer flapping issue is prevalent among people with small woofers that are tuned to go as low as possible. They have to move a lot farther to produce the same results as a large woofer." Only if the bass response is artificially boosted, as in your system. Otherwise, the smaller woofer will just roll off at a higher frequency than a bigger woofer. It’s even possible in some cases that the smaller woofer will be less likely to dance due to upstream perturbation, both because the smaller woofer can't respond and because of the enclosure damping, as in an acoustic suspension cabinet. This was mentioned earlier.
"Once digitized it is a simple matter to program in a subsonic filter with a very steep slope sidestepping the issue altogether." There is no free lunch in audio; fiddling with the signal in the digital domain will have a cost in SQ that some would not tolerate and others would find OK. Anyway, if one wants a subwoofer system, one can build an active low pass filter with a 24-db/octave slope out of discrete components that work in the analog domain, with potentially less of a sonic penalty. 24db/octave should be sufficient for cleaning up the higher frequencies.
