Don’t totally discount the DIY option before pressing the trigger on the Buttkicker.
I have two 50w bass shakers connected to 2 separate amps connected to my chair and it is pretty good. The two amps are probably overkill but offer easy expansion for more speakers if I want. To be honest my knowledge of electrics, amps and resistance was a bit sketchy but it was fun getting it up to speed and setting it all up. For the record I also had to buy speaker wires, a USB soundcard and bass shaker software so in the end it probably wasn’t that much cheaper than the Buttkicker option.
The easier option, though, is the Buttkicker and was an option which admittedly tempted me for a while.