In my house I have three Buttkicker LFEs and 3 of the 1000W amps, two Buttkicker Advance with wireless amps, four Aura Bass-shakers (not Pro) and four eBay-bought K-Woon shakers.
The Auras are the same internally as the Pros, except with less efficient cooling, so they're rated at only half the power. Both these and the K-Woons go down to 20 Hz and the Buttkicker products go down to a theoretcial 5 Hz (I've successfully tested the LFEs at under 10 Hz). I can tell you that even 20 Hz is a great feeling and in most cases, the Aura and K-Woons aremore than adequate for giving an immersive sensation.
The K-Woons, in particular, are fantastic value and blow away the Auras for power.
The benefit of a 20Hz rated shaker is that one can use a normal HT receiver if required. For this, I have the option of using my old Yamaha receiver, which has a 20 Hz to 20 KHz frequency response. Added benefit is that I can use the remote, or program a universal like a Harmony to control the shaker intensity.
I really do like the quality of the buttkicker products, though. Using the platform provided in the wireless kit, I was able to get one foot each of a fairly heavy couch and loveseat combo on to it. The buttkicker Advance with the 300W wireless amp was easily able to shake the seats pleasantly in movies, even with 5 people in total seated on them. My goal is not to cause violent shaking, but rather to enhance the rumble of bass in movies, and this was done just right. I now have an LFE and 1000W amp there, and it can do all that the Advance could, of course. I have the dial up at about 1/3 and typically watch movies at -20db on my Onkyo 5007.
All this to say that I think you'll be happy with whatever solution you choose as long as it's mounted properly. A cheaper 20 Hz shaker can be a tremendous addition to your set-up, and you don't have to opt for Buttkicker stuff, although they do make a great product.