If you HAVE to use one, then you have to- but you really should find the real reason for the ground loop. You are just putting a band-aid on a big wound.
Noise filters degrade the entire signal from your HU.
Make sure your amp grounds are clean.
Make sure your ground from your Alt to Batt to Chassis is large and clean.
Look for wires next to RCAs and Isolate them from one another.
For your HU, you really shouldn't use the factory ground from your vehicle's wiring harness- run a better one.
If you have a Pioneer HU, they are bad about having ground loops.
To fix it, wrap a small gauge wire around the shielding of the rcas where they connect to the HU and then run those wires to ground.
That will handle most RCA noise.