Just for the sheer fun of it, I've tried the Subkick on Kick Drum, Floor Tom, Bass Amp, and Heavy Guitars.
IMHO, it's completely useless for anything other than kick. But it sure is great on kick.
We have a few NS10 woofers (aside from the ones in the NS10s), and I prefer them to the subkick, but any sonic benefit is far outweighed by the Subkick's ease of placement.
As far as what other woofers you could use, just about any woofer that isn't
too stiff should work. I imagine a smaller speaker (8" or less, with a small voice coil) would probably be a little more manageable, both in sound and placement. What about something like
this?