While I am no 'expert' by any means, I have successfully installed literally dozens of car stereos in my own and others' vehicles, I continue to fight alternator noise in my boat. I've checked for ground loops in all 3 amps, the electronic crossover, and the head unit...all seem to check out fine. I have 8 gauge wire running from the battery to the head unit, running into a power distribution block and ground block. All the components have separate power and ground wires running to these blocks. I have high quality twisted pair RCA cables connecting everything.
All power and ground wires run about 2 feet away from any RCA cable.

And still....BAD alternator noise.

I am wondering if I am just not getting a good ground, even though I go straight to the battery? A boat is not like a car of course, in that there is no metal chassis to provide good grounds.

About all I haven't tried is to ground the RCA cables themselves on their outer rings (a process I don't have a clue how to do), or to see if perhaps the antenna is somehow causing a ground loop, but I don't see how this is possible since it is mounted in fiberglass.

EXPERT help would be appreciated.