Putting it bluntly, it's beyond me why anybody would wanna use a sub in connection with a bass guitar... In it's lowest working area, the fundamentals are weak anyway, & the instrument actually only works because of the harmonics (overtones) - this is true for by far most bass instruments, btw. So - if you wanna build a box SPECIFICALLY for bass guitar use, stick to one box with one type of unit (maybe with an added high frequency unit, if you find that necessary). One can then choose if it's gonna be one or two 15", two 12" or even four 10", & by prober box tuning having the box work down to the wished for frequency - the lowest note a normal bass guitar hits is ~42Hz, but as mentioned, this fundamental is weak, & a box with a considerably higher cut-off can still work very well!I'd like to mention that there was a discussion on the HE forum a while back, mainly among bass guitar players, concerning how low they should go. Most agreed that the boxes didn't have to go very low, some even said that it actually tended to muddy the sound if they did, especially at gigs. It was also mentioned by a poster that the best sounding bass gear he ever heard from a stage had a roll-off already below 90Hz - more than one octave over the lowest fundamental...
Just my two cents, Gabriel :-) Good luck with your project!