ButtKicker has resolved that problem, finally, with their own power amplifier that not only has the watts, but the input sensitivity as well.
The ButtKicker BKA-1000-4 is a digital switching amplifier capable of delivering 1,100 watts into a 4 Ohm load (the impedance of a ButtKicker). It is specified as also capable of delivering 2,200 watts into 2 Ohms should you want to connect two ButtKickers in parallel (e.g., one on either end of a couch, or one under a chair and another under the couch).
Digital switching amplifiers are not really perfected for general use throughout the audible range, although a few companies seem to have had success. On the other hand, switchers are perfect for subwoofers and shakers, because they work fine in the low frequency range.
The BKA-1000 is designed specifically for the ButtKicker shakers, and operates from about 5 Hz up to several hundred Hz (see On the Bench, below).
The front panel has a Power On/Off toggle, Volume knob, and two filter switches. One filter switches in a Low Cutoff filter at 25 Hz, so signals below 25 Hz are filtered out. The other is a High Cutoff filter that is variable between 40 Hz and 160 Hz. This can be used to keep out everything above 40 Hz, or whatever setting is desired.
The rear panel has an RCA input with a sensitivity of 160 mV, and a 1/4" phone jack input with a sensitivity of 1.25 V (the kind of thing used in road show amplifiers, and since rock groups often use the ButtKicker to feed tactile response on stage where the sound is too loud to hear the beat, they wanted to make sure the groups could use it there).
There is one set of speaker binding posts, a 115 V/230 V switch, and detachable grounded power cable receptacle.
The speaker output is marked "Bridged", which means there are really two amplifiers in the chassis, and they are bridged to give more power. It also means you should be careful not to ground the "-" speaker terminal, since it is not a ground connection.
I used the BKA-1000 with my ButtKicker shaker connected to a plywood platform on which my LazyBoy chair sits. This particular ButtKicker has been replaced with a newer model that is smaller and has speaker connectors already attached. This gives a bit faster response, since the steel cylinder inside is not as heavy as in the original model.
The amplifier delivered as promised, with an almost unlimited supply of power. I did not need any additional preamplification to drive it. The only issue might be with your circuit breaker, as 1,000 watts eats up half of a 20 amp circuit. The BKA-1000 did not get hot, even at full output, a testimony to digital switching design. They really are ideal for this kind of use.
My platform sits on a rug, which then is on top of the main wall-to-wall carpet. The ButtKicker is mounted at one end, which makes the platform move like a lever. More efficiency can be had by using rubber spacers that hold platforms off the floor. ButtKicker supplies these at a very low price.
On the Bench
The BKA-1000-4 delivered a maximum of 62 Volts into 4 Ohms at 30 Hz, which is 961 watts continuous. This is a little below the specification, but still plenty of power for shaking chairs, couches, or any other structures. I was not able to test it at 2 Ohms.
The measured frequency response is shown below, with various switches turned on or off. The first one is with all filters off. The response rolls off below 25 Hz and above 200 Hz.
This next graph shows the response with the Low Cutoff filter switched into the circuit. You can see that the response now is sharply reduced below 20 Hz, while the upper response is unchanged.
When the High Cutoff filter is switched in (Low Cutoff filter off), and the dial is set to 40 Hz, the response curve shows a steep decline above 40 Hz.
With the High Cutoff set to 70 Hz, the response now rolls off above that frequency.
At last, the ButtKicker can be driven with a power amplifier that is designed specifically for it. The BKA-1000-4 has plenty of power, enough sensitivity that you don't need an additional preamplifier, and has full bass management controls. Best of all, at $500 for 1,000 watts, it is very affordable.