Try a divider network between the empeg's outputs and the nakamichi inputs: I've not done the sums in my head, but this should work ok, assuming a high input impedance in the head unit.

empeg output (center) -> 5.6k -> nakamichi input (center) -> 1.8k -> empeg ground (shield).
Obviously you also need to connect the ground of the empeg to the nakamichi ground. Does the unit work ok if you turn the empeg output down? If it doesn't, then the problem is elsewhere (eg: have you tried fitting the grounding leads to the Nakamichi input and grounding them to the Nak's case?)

Hugo