The empeg comes with 2 sets of short male-female RCA leads, with a shared ground connection between each left-right pair.

If you still have both of these in the box, then they didn't use them :)

These may well need to go between the empeg line-out and the nak line-in, with the grounding flying lead being connected to the nak's ground point/casing.

Definitely worth a try - ask your installer if they tried this.

Hugo