Elliott Sound Products –– an amazing, no-nonsense resource for audio circuit schematics and information about DIY audio generally
Zach Poff –– New York area media artist, educator, and maker-of-things. Awesome DIY resources, especially for field recording equipment.
Black Lion Audio –– should be well known to any pro audio folks, renown for their audio modifications and, more recently, their custom preamps, master clocks, and converters (?????)
Revive Audio –– another great audio modification outfit, based in Idaho.
Daniel Feldstein (a.k.a., Daniel-dial-a-tone) –– one of the original engineers at Black Lion audio, does spectacular custom audio work.