Other people have put the same system in and made it work.
Did you use the Technic Harness and wire it according to Technics instructions?
The JL Amps claim to accept balanced inputs yet have RCA instead of twisted
pair Input terminators . I assume the shields on those are not a common ground. What about the level switch for Inputs it is in the right position.
I would try a cheap LOC before buying an MS-8 though you shouldn't have to do that.
http://www.e90post.com/forums/showthread.php?t=368847
The guy in the above thread(towards the end had some noise issues having to do with input levels.)