I disagree with the comment that is often said "These cabinets were "designed" to work together".
Oh yeah-in what way?
I don't know, but it says so right there on the spec sheet for the sub. It specifically mentions the matching top.
I'm guessing that they may have used the same 15" woofer in both boxes to keep the voicing similar... who knows. If Chuck said that they go together, then I'm guessing that they may have done something to specifically match those loudspeakers.
However, I generally agree with you completely Ivan, there is no really good reason that you can't use any subwoofer with a given top, as long as you can get them to play nicely in the crossover range, and with the availability of DSP for processing crossovers these days, it is WAY better than it was in the bad old pre-DSP days.