I found the SWARM by Audio Kinesis (Duke LeJeune) to work very well in evening out bass modes and giving me some low slam without mucking up the bass. From his website:
"The Bass System
The main obstacles to natural-sounding bass reproduction are the inevitable room interactions - which impose large peaks and dips on the bass response. By using multiple subs spread asymmetrically around the room, each sub will produce a unique peak-and-dip pattern at the listening position. The combined average of these unique peak-and-dip patterns is much smoother than any one of them would be, resulting in more natural-sounding bass with excellent pitch definition. The Planetarium bass system also avoids the subjective slowness of most subwoofer systems by taking room gain into account. Typical in-room extension is to 20 Hz, but the main advantage of this approach is bass quality, not quantity.
Planetarium Alpha and Beta modules available separately; bass system sold separately as "the Swarm"."
"The Bass System
The main obstacles to natural-sounding bass reproduction are the inevitable room interactions - which impose large peaks and dips on the bass response. By using multiple subs spread asymmetrically around the room, each sub will produce a unique peak-and-dip pattern at the listening position. The combined average of these unique peak-and-dip patterns is much smoother than any one of them would be, resulting in more natural-sounding bass with excellent pitch definition. The Planetarium bass system also avoids the subjective slowness of most subwoofer systems by taking room gain into account. Typical in-room extension is to 20 Hz, but the main advantage of this approach is bass quality, not quantity.
Planetarium Alpha and Beta modules available separately; bass system sold separately as "the Swarm"."