If you head is permentally magnetized it won't work at all. In my experience residual magnatizum on the heads or other metal parts in the tape path affects the tape hiss first, even before any significant dropoff of highs. If not properly demagnitized the hiss will sound really grainy and eventually will become louder than normal. This will happen if your bias is wrong also. What a lot of the time people don't tell you is that you really have to be close in, around an eighth of an inch, to actually demag anything well and the tape guides and capstan are a lot harder to demag than the heads. Also the tascam demagnitizer thing usually is only strong enough for cassettes or small format reel to reels and have a really hard time demag'n tape guides. For serious demagnetization check out r.b. annis, but you will have to be more careful with their demagetizers since they are much stronger and can possibly overheat in operation, permentally screwing up your heads. Also by the time heads to be magnetized enough to screw up your previously recorded tapes everything will sound like crap anyway, but make sure everythings cool anyway before you put on those lost Beatles' masters.