I've noticed that my system also has a VERY SLIGHT amount of tweeter hiss when the car is powered on.
BUT ... with the car running, there's enough ambient noise that the hiss is undetectable, so I don't feel that it's a problem at all (for me). I can't hear the hiss if idling in the garage, so you can bet that on the road it's a non-issue as well.
On my system, the hiss is definitely proportional to the gain, but I've got the gains rather low, so that resolves that issue. (The system will still get uncomfortably loud with the gains set low)
Is your hiss bad enough to hear it if the car's running? Or are you checking it with the car powered up but not running?