Re: ground loop isolator
A ground loop isolator uses coupling capacitors that allow AC (audio) to pass, but block the DC path. A capacitor is also a hipass filter, depending on its value and the value of the impedance it sees as a load. Your case sounds a bit extreme, but it's quite possible to lose low frequencies through it if the load impedance you plug it into is low.
Make your own ground isolators. Get a pair of short M-F RCA wires from RS. Cut them in half. Strip the insulation back, and reconnect the center wires but not the shield. Tape them up nice and secure.
That's what I'm using. It's only a partial isolator, since it doesn't isolate the HU preout driver impedance, but it's enough to kill most alternator whine without losing frequency response.
1999 Buick Park Ave Ultra
Head Unit: JVC KWHDR720
EQ: Audiocontrol EQL
Front Stage: Hybrid Audio Technologies Imagine 5.25 mids, Morel Maximo tweets
Sub Stage: Boston G3 10", 0.5 cu ft sealed
Amp: PPI Phantom 900.4
Rear Fill: Boston SE953 6X9's