It's kind of a long story that I'll have to go into in more detail at a later time, but I've now "supercharged" the Buttkicker installation on my floor joists. Since a vibrating floor effectively acts like a huge infinite baffle transducer, I was curious to see what it does to my bass response. I have the BKs eq'd to roll off significantly above about 12hz, so upper end response isn't affected. But here are some quick measurements I just took to quantify how much they contribute. Green line is with the Buttkickers turned off (so it's just my sub's response), while the blue and red lines are with the Buttkickers active, at two different opposite polarities (to see which "phase" mixes best with my sub):

http://home.comcast.net/~consumes/bass/comparebk+bkreversed.jpg

Ignore the overall response, as I've recently rebuilt my sub, and I haven't done a final Audyssey run (and there's NOTHING I can do about the big null @ 11hz). The important thing to look at is what they do relative to just the sub by itself. As you can see, at 4hz, with them wired one way they are cancelling out my sub's response by about 2.7db down, and wired the other way they are helping the response by about 2.7db. Unfortunately, they don't have the same phase response as my sub, so I can't wire them such that the effect is additive at all frequencies, but I can at least pick one that helps more than it hurts.

Anyway, I thought I thought some might find it interesting to see the acoustic effect of a floor installation of Buttkickers. :)