Before you buy another amp there are a few things you must realize...
1: The Buttkicker LFE (the top of the range) has a frequency response curve that goes all the way down to 5Hz.
2: Lots of movies also have LFE's (low end effects) that go way below 20Hz, some even down to 10Hz.
3: Very few amps have a frequency response curve that is effective below 20Hz. With most amps, the roll-off from 20Hz is often quite steep.
4: There are amps available that do have response curves that go to almost DC (0Hz). Most, but not all, are expensive, but you have to search for the less expensive variety. PA amps are the best source because anything in the "hi fi" range is ludicrously expensive. I could buy a seat on a moon trip cheaper.
Some amps also have a switchable crossover that will allow output below 20Hz. Using this option with speakers is obviously not a wise thing to do, but a definite requirement for the BK LFE.
A thing about running these devices in series and parallel...
Be very mindful of the current, as well as the inductive and resistive loads you placing on the amplifier. Very few amps will stand the strain of running high output into low impedance loads, even though they mention these loads in the spec sheet when referring to output power. 3 sets of 4 ohms units wired in series gives around 12 ohms (but not precisely). 2 sets of 12 ohms in parallel will result in a load of around 6 ohms, so in impedance terms with the BK's running at 4ohms you are probably safe. But the actual current load on the amp is pretty high and as you discovered, will eventually give up. Any amp with a power supply weighing less than a ton will be unsuitable (if you get my drift).
Because high power PA amps (Fostex and Behringer) can be more moderately priced compared to other alternatives, this is the obvious way to go, but if they roll-off steeply at 20Hz, then you will only realize about 80% of the potential of BK's. But you will have to suffer the noisy fans they use (or change the fan).
I am one of the lucky ones who had a 2x300w Fostex available (delivers around 700w mono bridged into 4ohms) when I bought my first BK LFE and I love it.