I tried my spare (with the caps) and to me it seemed to be reduced slightly with the spare...maybe...

It not likely to be related to the capacitors then. You need to start going through the ground loop elimination steps for your amp installation.

When I get the time, I'll try to use another ground for the Empeg. Right now I'm using the car's ISO-connector.

Try attaching a thick ground wire to the screw hole on the back of the sled, and finding a proper ground point on your car's frame. Tips on the proper ground point can be found here.