JRF Magnetics are great, I'm sure, but their site really only is interesting if you want to relap or replace your tape head. But a site that is ALWAYS interesting for anybody with a tape recorder is Magnetic Reference Labs: http://home.flash.net/~mrltapes/
What these guys don't know about tape recorders isn't worth knowing. Loads of information, and maybe not that accessible, but it's there. Read, read, read!