Aha - makes sense - thanks Penance

Anex - demags are useful to have; I've got that Nakamichi one that's for both open reel and cassette - although I still can't suss out how to get into my Akai CR81D 8track cartridge deck to demag that...

Basically, magnetised decks will create more noise on playback, potential partial erasure to tapes, and degrade recording quality.  It's always useful to demag the heads/capstans/guide pins if you can.