I don't know if the zoom has an EQ setting but that's your solution. Just lower the frequencies that are causing the harsh sqealing while leaving alone the ones that produce harmonics. Another thing I've found helpful (on a Marshal stack at least) is an attenuator inline. That allows you to use more of the amps natural overdrive capabilities while still keeping the volume down.
Hey you kids! Get outta that Jello tree!! :mad: