The head unit seems to have a good line output. I don't remember it being an issue in cars I've worked on.
The hiss you hear is from turning the gains too high. You need to set the gains correctly among other things.
First, set the crossover point on your door speakers to a value around 100 Hz, high pass. This will allow them to play louder since they won't be playing bass and distort at lower volume levels.
Next, make sure your sub gain is not too high and that it is also crossed over at a value around 100 hz, low pass. If the gain is too high you might be trying to crank up the volume which makes the sub play louder as the door speakers play louder and it's a no win situation.
The door wiring if you used the OEM wires of white and orange are sufficient, not optimal. You shouldn't have much of an issue with those wires. But if you ran some tweeter wire up to the wires on the harness or to the crossovers, it would help you to rewire that. 16 gauge should be sufficient for any of that wiring.
You did not mention what kind of components you are running. Sometimes those crossovers are a bit tricky with extra outputs, switches and so forth.
