Honestly, you're probably right. I don't have a great amount of experience with pro audio drivers, but have read a little bit on their capabilities and it seems like they'd be an alternative to the usual, XXX, Ava, TC 4HP, etc etc.I don't know of any pro audio subwoofers (i.e. high efficiency) that would be worth two shits in a bucket in the amount of space he has to work with.
For SQ, the "older" XXX and the Ava would be decent choices. Hard to find, but decent choices. Neither of those are very easy to locate, however.
Oh, it was definitely a good general recommendation. Many pro audio drivers are excellent from a sound quality standpoint (like the IDW's). Just that darn Hoffman's Law gets in the way when small enclosures are required (and 4cuft is pretty darn small for a 18"). High efficiency and small enclosures only work well together if low frequency extension isn't a concern, which isn't the case here. So, for this particular instance, he's going to want something with "relatively" low efficiency, so that he can still have his low frequency extension in a moderate enclosure size.Honestly, you're probably right. I don't have a great amount of experience with pro audio drivers, but have read a little bit on their capabilities and it seems like they'd be an alternative to the usual, XXX, Ava, TC 4HP, etc etc.