Man, I got that ground loop isolator in the mail today, installed it, and it works! If your still having problems, and still cant find a solution, get one of those! Only cost me $10 on eBay... Might even be able to get them at radioshak...

And WiLL, yes filters will filter out some sound, so the amp gets less of a signal, but its the amps job to amplify it... It can take a weak signal and make it much louder than it actually is...