Unfortunately, all but the most expensive subwoofers and home-made efforts produce hideous amounts of distortion - I've yet to hear a single "subwoofer" from any mainstream manufacturer (excluding SVS) that couldn't be located instantly in-room.
A high amount (more than 20%) of harmonic and intermodulation distortion is a fact of life with any speaker, subwoofers more than most. Spending time and money eliminating this problem has happened in many studios and is attempted by many audiophiles.
Why, then, does it seem that budding musicians are apparently having it drilled into them at university or wherever, that a subwoofer needs to be a little box in the corner of the room and you need to be able to hear the noise it makes separately from all the main monitors?
I have a feeling this sort of thing could account for much of the bad mixing that's happening in studios today.
X-posted to my own journal, hope you don't mind =)