Your Buttkicker 2's have a vastly different drive power requirement than the Aura shakers you purchased.
The Buttkicker's need somewhere between 400 and 1000 watts to drive each one
and they recommend 600 to 800 watts. They clearly need something far more powerful than a yard-sale, beat-up, stereo amplifier you can find for $20. Instead, they need a dedicated, high power amplifier. Expect to spend a few $$ for that amplifier.
The Aura shakers however can easily be run on 20 to 100 watts of power. I have 4 connected to a 75 watt amplifier and it is plenty.
Now, as brickie said, it is important to present a load your amplifier can handle AND also to wire the shakers in such a way that they all shake the same amount. I've seen many "suggested wiring diagrams" where the total resistance was fine, but where most of the shaking was being done by one of the shakers and the others left with a small fraction of the power.
For six shakers you can wire three in series. That will give you a resistance of 12 ohms. Then wire a second set of three in series. It will also add up to 12 ohms.
Finally, wire the two series strings in parallel to your amplifier. Two 12 ohm loads in parallel will result in a 6 ohm load, just about perfect for most amplifiers.
Attached is a drawing illustrating what I tried to describe in words. It assumes you have a monaural amplifier. If you have a stereo amplifier, use a "Y" adapter to feed both the left and right inputs from the LFE output of your main receiver and wire one set of three in series to the left output and the other set of three in series to your right output. They will each represent a 12 ohm load and be very easy for it to drive.