The pop is mainly a capacitor discharging in the diff output circuit - one thing to try is to open up the plug end of the grounding leads supplied with the empeg and fit a 1k resistor between signal and ground: the output of the empeg is 'stiff' enough (current-wise) to deal with this load without affecting audio quality, and it reduces the size of the power-off pop. Try it on one channel first and see if it helps.

Wether you get this or not depends on various parameters like how the amp is powered etc.

Hugo