Just to add a bit more of clarification, it's important IMHO to remember that the *purpose* of a subwoofer in the studio is not necessarily quite the same as it is in a listener's playback system.
In a surround sound system the usual idea is to provide real bottom end, to provide almost as much feel as sound. If there is a bit of hole or inaccuracy in response at somewhere the transition from bass to sub bass, it's not so much noticable or necessarily all that important to the average surround listener. If they needed accurate bass reproduction in all channels, they would depend upon their mains to supply most of it; the sub is there to put the SUB in subwoofer.
Audiophiles will rightly disagree, but audiophones are not your average surround users. They are aslo the ones most likely to tweak their crossovers to fit their system, and not just follow a preset standard.
(In the car, the only purpose of the sub is to annoy as many other people as possible and generally make life on the planet just that much more miserable. So we'll ignore those.
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In the studio, however, there are two semi-distinct purposes for a sub; one is if one is mixing in and for surround and needs to sub to be able to have a full surround system to mix to. The other purpose, and the one I believe we're mostly talking about here, is to extend the low end frequency coverage of the miser's monitoring chain so that they can accurately provide a balanced mix. This is especially important for those whose main stereo monitors are not all that good in the bass and low bass frequencies, the ultimate examples being those with computer-grade speakers, low end budget "studio monitors", or old school nearfields like NS10s or Auratones that lack in bass almost by design.
Those looking to set up studio subwoofers for that second purpose should - IMHO - be looking to smoothly extend their main's response to the best of the sub's capabilities more than they should be looking to break the china in the dining room. It's subwoofer with the emphasis on the woofer, not the sub. If you can get them both, great. But a sub that gives you great sub, but doesn't dovetail with your mains to give you a smooth continuum of frequency response, is going to give you trouble mixing at some important bass frequencies that can be troublesome enough to begin with.
This is why the "standards", which were developed almost entirely with entertainment playback systems in mind, shouldn't necessarily always apply to the use of subs in the studio, and why an 80Hz crossover "rule of thumb" in the studio can have so many exceptions as to render the rule virtually impotant.
Of course, IMHO, the ultimate solution is not to get cheap mains and try to fix them with a sub, but rather get decent mains to begin with. But there's a whole lot of "your situation may differ" in that.
G.
