Hissing noise in my speakers...help!!!

Hi all! I recently upgraded the speakers and amplifier in my BOSE system on my Mazda3. The amplifier is a Alpine MRV-F540. I decided not use a LOC, instead creating a harness from a metra adapter which converts the high level speaker inputs to RCAs. The head unit was kept stock and I want it to stay stock, if possible.

Everything sounds great, but I have one nagging issue. I have this persistent "hissing" sound coming through my speakers. There is no crackling and there is no whine - it is pure hiss. The noise is mid to high frequency in nature and becomes louder as the gain and volume controls are turned up. It is not related to incorrect gain settings as the noise is still present and noticeable when the gain is all the way down. The noise is not affected when the car is turned on.

I have tried many things including rechecking all the grounds, adding a ground wire to my head unit, checking and going over my harness, using a PAC-AOEM Harness with built in gain controls to bypass the input wires running in the car, and using different RCA cables. None of these things had an effect the hissing noise in the slightest.

The one thing that DID eliminate the noise was when I hooked up my IPOD directly to the amp's RCA inputs. This is the only thing I changed; The speaker outputs were still running through that adapter I created. During the "IPOD test", I paused the music, turned the gain all the way up and maxed the volume on the IPOD. There was not one hint of noise after doing this.

This makes me believe that the noise is coming upstream through the amp and being induced through the RCA cables somehow. Is it possible that my HU's power supply is leaking noise into the RCAs, or that my amp is having compatibility issues with the HU? How can I test for these things? What "type" of noise is hiss (IE - what is it usually indicative of?) I have no idea what else to check to pinpoint the problem, and any help would be appreciated. Thank you!