Here's a page with various links to helpful information, for better understanding the what, why and how of loudspeakers and other audio related equipment.
diyaudio.com by fanatics, for fanatics. The best worldwide forum for audio enthusiasts.
Hornresp.net Software for loudspeaker design and simulation, by David J Mcbean
Quarter Wavelength Loudspeaker Design, by Martin J. King http://quarter-wave.com
Elliott Sound Products - Professional results for the Do-It-Yourself enthusiast
Rane note on: Signal Processing Fundamentals
Rane note on: Bessel filters and It's relation to others