I've stumbled upon the whole Infinite Baffle Sub idea, and I can actually do this since my garage borders my HT room.
I haven't tried it, but I've looked at it seriously. The theory is excellent: you can use a bunch of drivers and drive them all very gently, which keeps them all in a well behaved, linear range. Enclosure resonance isn't an issue if you have a large enough ("infinite") space behind. Lots of advantages.
There are some down sides, too. If the space behind is too small, it will have a resonance that falls in the range of bass frequencies, and this could play merry hell with bass response. That's generally not a good thing, and you can end up having to treat that room too, to keep big bass peaks and nulls from being a problem. It works fine with an infinite space or even a large irregular attic, but your garage is probably of a size that's going to cause problems. And putting acoustic treatments in a garage might or might not appeal.
Second, anything you play in your HT is going to leak straight into your garage, and vice versa. Especially the bass! Half your bass energy is going to fill your garage, and bass carries a long, long way. You might find your neighbors gets tired of hearing only the bass line to your favorite music.
Third, the usual way to do this is to throw in a number of woofers, not just one. You can get lots of bass that way, but you will also end up with a complex bass pattern, tricky modes and nulls and phase questions, in the room that's hard to control and treat. If you go the multiple driver route, make sure the woofers are arranged symmetrically with respect to your room, and try the speakers out in those approximate positions before you cut drywall.
Last... whether it's one woofer or several, you're making a hole in your wall. If you don't like what you did, you get to patch your wall again ad cut more holes. Whereas with a subwoofer, you just move it to a new part of the room, or send it back. No muss, little fuss.
If you love experimenting and cutting and patching drywall, and want really prodigous bass, it can be a good way to go. I might have given it a shot if my room had allowed it... but given the complexity, I think I'm glad things went otherwise. Boxed subs have plenty of reach and power; even in a *very* large room, I just about never turn mine up past 3/10ths.
