I assume you are running a separate (fused) power lead to the empeg itself and verify you have a short direct connection to the chassis gnd somewhere in the dash. Avoid grounding near A/C units and dash fans as well. If you already played all the amp and head unit housing gnd isolation games then supply noise is a likely source. Do _not_ single point gnd your amps and head unit and crossovers. Every piece of equipment should have a separate short and healthy gauge gnd (based on it's current consumption). Also, run clean power directly from the battery. You don't need a different run for each piece, but keep the high current and low current stuff on separate runs. Fuse everything, don't catch your carpet on fire because you were cheap and/or lazy.

Additionally, do NOT get those quintuple über shielded RCA interconnects. You want to run unshielded twisted pair in a car (remember the entire car chassis is gnd) no matter what the sales idiot or installer tells you. Using shielded interconnects adds an additional path for gnd currents. You aren't shielding anything, the majority of noise in car audio is conducted not radiated. Considering the entire car is a conductor, people often miss this point.

#080000512 green 12GB