At first I had no idea what this thread was about; now I realize. Not wanting to be a wet blanket, but I hope you all realize that it's total self-serving pseudo-technical bulls--t when for example
the Web page for this product says, "We took a 6.5" speaker and shock-mounted it into a 10" drum shell and reverse-wired it to an XLR jack to convert the speaker diaphragm into a microphone diaphragm. This allows Subkick to pick up the low-end that a normal microphone can't."
I mean, it's always fun and an instructive prank/science experiment to try using speaker elements as microphones. The results can be surprisingly good sometimes, considering the randomness of it all; in a pinch some dynamic headphone elements can even be used for recording. But the central premise of the SKRM-100's marketing statement is simply wrong, since the size of a microphone has nothing in particular to do with its ability to pick up low frequencies. There are measurement microphones 1/8" across that have ruler flat response down to 1 Hz, for example, while many revered "vintage" large-diaphragm microphones have nothing much but stand rumble and wind noise below 30.
Size matters greatly in a woofer because it has to transfer power and move large volumes of air; if a woofer didn't have a large cone it would have to "excurt" an impossibly long distance to pump audible amounts of low-frequency sound energy into a room. But microphone diaphragms simply respond to vibrations with
very slight movements of their own. They're like the sails of a boat; they don't have to provide the wind! And the more you weigh down the moving system of a microphone, the more you screw up its transient response. So I would think that particularly for a drum microphone where the character of the transients is so much of what matters, you really, really wouldn't want a microphone with this approach or anything remotely like it.