Well after finally getting a chance to install my new C1100.4 I had a terrible hiss with it that only got worse as the gain went up. So far I have tried swapping everything around the amp to eliminate it them from the problem except new rcas and I haven't tried it in another vehicle. New Rcas will be here today, shielded and all. I have swapped HUs I have Tried both sets of channels Amp is ground directly to battery Gain High or low high. Tried a different amp and don't have that issue (I have low level noise but I believe its a source unit problem) Amp does have the HPF on and set to 5khz, which is what I need to actively run my tweeters. Other amp which I have no problem is a ID q450.4 Ill be trying it in a different vehicle today and new rcas after that. Any advice on getting this narrowed down to the amp would be amazing.