Might be able to place a diode (in the proper polarization) between the amp's + and - leads to counteract the induced inverse voltage produced by the collapsing magnetic fields of any coils, relays etc., when the amp is turned off.

Most modern automotive relays now have this diode incorporated into their design to prevent damge to automotive electronic systems. Check with your local automotive sound store to see if this might help, and also how to wire it correctly.