there's something martin or someone else will add about dc coupling and
gaussing up the heads, which I forget at the moment.
If you unplug the headblock (only when the power is off please) then you can, by accident, put DC across the heads by plugging it straight back in. This can magnetise the heads! The preamp stores a charge for some time so the input should be shorted for a short while before plugging the headblock back in.
Norm