I don't know what a Shefield demag disk is. Are you talking about this:
https://www.amazon.com/Sheffield-Xlo-Test-Burn-CD/dp/B000025RLX
Although that seems like an interesting, if wildly overpriced, product I can't imagine why it would fix anything in the speakers.
https://www.amazon.com/Sheffield-Xlo-Test-Burn-CD/dp/B000025RLX
Although that seems like an interesting, if wildly overpriced, product I can't imagine why it would fix anything in the speakers.