The two most common reasons for hiss is either a grounding issue, or a gain staging issue.
To troubleshoot if it is a grounding issue, the first thing to do is to change the grounding jumper position on the unit.
You can find the position of that jumper here.
There are 3 different positions, isolate, ground and 200 ohm. You can try all three. If none of them get rid of the hiss, the next step is to double check how you have grounded the unit. AudioControl always recommends you ground the unit to the battery via the negative terminal. That option isn’t always available, so the next best way to ground it, is to the chassis of the vehicle. When you do this, you want to make sure it is grounded to that actually steel chassis, a lot of newer vehicles chassis are aluminum, and if you grounded to aluminum, that can cause problems. When you ground to the chassis, you also want to make sure there is no paint on the chassis. Paint can interfere with getting a solid ground connection.
You also want to try to have all of the aftermarket equipment grounded to the same point.
Another good step to confirm if it is indeed a grounding issue, is to take a temp wire and run it to the negative terminal of the battery. Using jumper cables is a good option, since you can just clamp the jumper to the terminal. You would then gather up all of your grounds to that wire. If the hiss goes away, then its a grounding issue and you need to re-configure your grounds.
If we’ve done all of that and you STILL have the hiss, then we would want to check your gain staging.
Meaning, the signal going into the converter may not be high enough, so you need to turn up the gain more, which will then bring up the noise floor more.