Bass by phase modulation.

I want to take two 50KHz synced tone sources, invert them, then phase modulate so that the difference between the two represents my actual signal.

I'm really seeking to produce a high power signal in the 0-50Hz range, but the phase modulation approach has three advantages. First, the high carrier frequency can be deadened and insulated much more easily than a pure low frequency signal. Second, the emitting devices don't necessarily need to be as large as a corresponding cone. Third, they don't need to be directly attached, as with bass shakers, making seating and such easier to manage.

The first problem with the phase modulation approach is the carrier signal; You don't want it to be within the audible range of any human or other hearing animal.

The biggest up-front problem, though, is managing the positioning of the interference nodes. Having a high carrier frequency solves part of the problem by reducing the distance between the nodes. The smaller the distance, the lower the likelyhood of a positive node being in one ear, and a negative node in the other. (Wouldn't want to scramble your brains, now, would we?)