Thanks for the input and help everyone... The problem may still be there but your suggestions have been invaluable in troubleshooting. I read in another thread where someone also had similar issues, with the hiss being very slight with the OEM system, and then putting in an aftermarket amp made the hiss much more noticeable (makes sense, the signal from the headunit is being 'amplified'). In you guys' experience then, could this be a bad headunit or bad grounding of the headunit?
If the headunit itself is good (assuming), would a new carefully routed twisted pair RCA line from the headunit to the amp improve the situation?
Technic, you mean like an MS-8 between the HU and the XD600? That would be on my wish list, but I might have to wait a while before spending more on car audio!
On a very side/off-topic note, I finally put on a set of performance blackout grilles. I can't believe I waited that long to get a set. They look stunning on sparkling graphite! That was the silver lining to the day, with all the noise issues going on.
Last edited by MJ777LR; 01-24-2013 at 08:25 PM.