Rcas pass thru problems

I'm running a rockford t600.4 for my mids, and I also have a rockford p300x2 to run my tweeters. I tried running rcas from the pass thru of the 600.4 to my input of my 300x2, but it makes a loud hissing noise through my speakers. My 600.4 runs my 6x9s really clean, but whenever I connect the rcas to my 300x2 it makes a loud hissing noise through my speakers and tweeters. Don't know what is wrong.