Specs for m&k subwoofers

I've been considering picking up a used subwoofer for home theater to round out my b&w 604's that cut off at 30 hz (-6db). I've seen some m&k's on ebay but I can't seem to have any specs for them. Any info on where I can find some specs? The m&k vx-4 is one that I have been considering.
I have been to the webpage and looked at the manuals....but no information about frequency response in the ones that I have looked at.