I have had this exact same issue for a while now. I am powering to 12" subs, splits and 6×9s from 2 amps and the stock headunit. My LOC is also the Audio control LC6.

I have been testing each part of my car thoroughly to get rid of the hiss.

Some interesting finds I have found:
Hiss is there even when car is not running (not caused by engine noise)

Plugging an alternative audio source (ipod) straight into the amp via RCA completely removes all hiss (hiss is induced BEFORE amps)

Disconnecting the LC6 and using the RCAs that are run down the side of my car for my alternative audio source (ipod) = Still no hiss

Running LC6i from my new aftermarket HU (infill G4) in the boot of my car powered from 240V with power inverter to 12V (So I'm not powering anything from my car battery except for my amps) and the hiss is there.

Therefor I assume my LC6 is producing noise. I want to use a Sven4 LOC to compare the 2 but I'm not sure how to wire the 4 speaker level input to 6 RCA output as the Sven4 only has 4 RCA out.

Anybody in Brisbane care to lend me any type of LOC to borrow