Can anyone recommend a good line filter?

[Boozer]

I am getting a ton of engine noise coming through my amp for mids and highs. I put on 2 cheapo line filters and they got rid of the noise for about 5 days. Now the noise is back and it is driving me crazy. Are there any good filters out there?

[Sotally Tober]

Are your RCA's and power wires close to each other? Try unplugging them and see if the noise goes away.

[Cat & Mice]

Make sure the Head unit and amps are on the same ground. If the two are on different ground plains then they will try to equalize the ground difference over the interconnects, this will create a hum or buzz. Also make sure both have clean solid grounds. This is usaully messed up in boats since they do not have a chassis to ground too.
Good Luck! These ground issues can be a pain to track down.
Also make sure you always use shielded interconnects so they dont act like an antenna and pick up a bunch of noise.
ST-had a good point about the interconnects and the power wires being close, the interconnects can pick up noise from the engine if they are not shielded and are close.
MP

[Shockwave 25]

I had the same problem and banged my head against a wall for 3 weeks. Then one night I decided to unplug the RCA cables from the head unit and touch the center prong with my fingers while the stereo was on. Major noice came out the speakers. So I ran the wires, as a test, through the cockpit and had the same result. Huh, bad RCAs??? So, what I did was make some custom length cables out of High End RG6 Quad Sheild Monster cables and re-ran them away from all power cables. The noise disappeared. I don't know if you would have the same results me, but maybe try running good RCAs. I thought my Street Wires were decent but boats don't have strong grounds like autos do.
Best of luck!