Stereo Help

Ok so I am in need of some help with my stereo setup. I had a modified stock system that sounded pretty good. The interior was lacking some power and I wanted to add another amp to help get my speakers a little more juice (and my sub amp died). Now I am getting feedback out of my 6 interior speakers. When the boat is not on or the exhaust fan is not running it sounds good. But as soon as you kick on the fan or turn the boat on you get a hmm out of all 6 speakers. Also my problem is I am out of RCA outs on my head unit so I need help there too. This is what I had and what I have now.

03 SAN Team 210 (old setup)
Stock tower, factory kicker interior, 12in sub in custom box.
KX300.4 – powering 4 interior, head unit powering 2 bow speakers.
Tower speaker amp…dunno what it is
Sub amp (died)

New setup
KX300.4 bridged on 2 channels for 12
Tower speaker amp
JL M6450 powering 6 interior

I moved all the old speaker wires off the 300.4 and put them on the JL amp, pulled the bow off the head unit. Routed new 4 gage power and ground cables to the JL amp. The JL amp needs a Y adapter or 2 sets of RCA’s going into it to feed all 6 channels. When I was using a radio shack Y adapter RCA I was getting terrible feedback. I changed it to just run 4 speakers and it is not as bad but I still get interference. I have no more RCA outs on my head unit, and I don’t have any outputs on my amps.

So the ? is, what should I try to get rid of this interference? My RCA cables powering the JL are thick monster cables. Could it be the thickness of the wires feeding the speakers them selves? That is all stock stuff, I am stuck here...and confused being i had no problems before.

thanks sorry for the long post