For anyone in the San Francisco Bay area, Roger Modjeski (Music Reference, RAM Labs) offers a course in hi-fi electronic design. In his course he teaches you electronic theory, and you get to build your own amp or pre-amp.
Rythmik Audio offers their sub system as a DIY kit, proving everything but the enclosure. To make shipping more affordable, Rythmik produces their sealed 15" woofer sub in a 3cu.ft enclosure. For those building the 15" sub with the DIY kit, he advices using a 4cu.ft enclosure, for higher efficiency and more output. His OB/Dipole Sub (co-designed by GR Researches Danny Richie) is offered as a DIY kit ONLY. Plans for the required H-frame are provided by both Rythmik and GR Research, and GR has a cabinet builder making and selling an easily-assembled frame to Danny's specs.