running a 12 and a 15 in the same cab through the same frequency range is not really recommended. the response could be quite unpredictable, especially if they share the same airspace. the "correct" application would be to cross the 15 and 12 to different frequency ranges. (like accugroove supposedly does) and contain the drivers in internally seperate indidvidually tuned enclosures. the problem is typically 12's and 15's cover so much common ground frequency-wise that you're better off ditching the 12 and using a much less expensive midrange driver that has a range far beyond the limits of the 12. the midrange won't have the bass of the 12 but with the low end being crossed to the 15 it's a moot point. the midrange will have better sensitivity above the point where the 12 peters out and it's cheaper, lighter, and smaller.
that said, making a 12 and a 15 work together can work for bass amplification, it would just require trial and error type development (or someone smarter than me, probably not too much of a feat for that) however if you're into studio-monitor type response, it's probably not the best design plan, but if it sounds good it is good. sell your current cab and find one that fits your sound. maby something like a 3-way 15 cab with a 15 extension. maby dr. bass or low down sound could hook you up. (or build your own)
