Wayne I don't know if you've heard of the ELF integrators that Bag End came out with several years ago, but it uses an integrator type circuit at the line level. It operates the speaker below Fb because the lower in freq. you go the higher the output, but they recommend that you use it in a sealed enclosure because with the system operating that far below Fb they use the air thats trapped in the enclosure. It acts a back spring for the speaker, so you don't send the cones flying across the room. You can use them with vented cabinets but they recommend you block the ports when using them.

Bill W.