Dana Music - Site from luthier Dana Sutcliffe offers a guide to match Ampeg amps and speakers correctly, as well as downloadable Ampeg manuals.
ESP Projects - Audio projects including amps and preamps suitable for musical instruments amps, effects, and some neat subwoofer/processor plans for bassists.
Fuchs Audio Technology - Design and hand-build tube amplifiers; includes model speficications and dealer network. Also offer a custom-build service.