I installed an Express RC XM roadie and wired FM modulator in my car yesterday. The problem is when I turn up the volume I get a bad crackle (almost sounds like a blown speaker). I know my speakers are fine and I have no volume controls on the Roadie since I am using the FM Modulator. The thing is when I turn down the Treble on my factory stereo (all the way to -6) the crackle all but disappears.
It sounds like you may be getting too much gain going through the two devices before your radio. (if I read that correctly), most crackle is in the treble range which is probably why it almost disappears when you turn the treble down. All you are doing actually is turning the crackle volume down, it does not disappear only goes down in volume.