Having done some research into building a bass cab, i got some useful links from the lovely chaps at em411.com
. Incase you don't know that's a site with a very active forum and blogs for electronic musicians (there's a rumour it's hosted by Bogdan Razynski...) amazing speaker design links page
This page tells you pretty much everything you need to know on speaker cab design, crossovers, calculations, room resonance. mint. College course
Some doccuments from someone's college course on building hi-fi speakers. line-level crossovers
Info on passive line-level crossovers, for before the amplification section. Filter design
Great tutorial on crossover design, including handy calculators for components etc. Basic crossover info
Great introductory page on how crossovers work.
Hope someone can find these helpful!