Weird stereo noise issue

Ok so after wiring up a neighbors boat a 2003 Xseries 205v hull we have a perplexing stereo noise issue .
I
First off little background on what's in it. He has a Syn 2 powering a pair of pro 80's. A Syn 4 powering 4 Polk UM 650 on the rear 2 channels and a kicker I think L10 or 12 on the front bridged. He has distribution blocks coming off a stereo battery strictly dedicated to stereo. 0 gauge from Bart to the block and 4 gauge from the blocks to the amps. The head unit and amps are all grounded to the blocks. Te Syn 2 is running directly off the head unit a Clarion M475 from the rear RCA. The front RCA goes to a PAC LC1 controller and then back to the Syn 4 rear channels for the 4 inboats. And the third set of non fade RCA from the head unit run to another PAC LC1 and then back to te Syn 4 for the sub. We originally used JL audio clear twisted cables 18ft to run from the head unit to the 2 LC1 located at the drivers helm and got a noise. He then ordered up some xScorpion quad shielded RCA cables and still got the same noise. It has to do with the RCA runs to the PAC LC1 we think because when eliminated the noise is gone.

The noise is Only in the inboats . Whether we switch outputs from the head unit front or rear we thought maybe blown channel, or swap lc1 controllers. When the lc1 is at full open volume there is no noise, when the lc1 is a full close no volume there is no noise, but when the lc1 is around half volume we get interference when the boat is running that is like crackling and crackles casters with the Rpms going up. The weird thing is is again at full wide open on the lc1 no noise at all and at full close meaning no volume to the speakers no crackling?


Any thoughts?

He first thought maybe interference from under the dash with all the electronics but before I upgraded to a 420 I ran mine the same exact way with zero issues although with kicker amps.