[geekwithfamily]

I guess you could use the second channel on your current BFD to alter the shaker's response. Because you're not dealing with sound waves, just electric pulses and couch vibration you could boost the frequencies between 16 Hz and the peak at 40 Hz. The shaker is tuned for movie explosions, but you'd think the lower it got the more shaking your butt would receive (maybe the shakers are tuned that way because the magnets' throw cannot accommodate frequencies lower the 40 Hz, the Y scale in the chart Sonnie posted is not amplitude but pounds of force).

The only issues I'd worry about boosting the signal would be blowing out the shaker, falling out of phase with the action on screen and hopping your couch across the floor (it's hard enough to keep your head in the sweet spot as it is).