Yes you can control it with the gain. The good news is when you overpower systems like that you get to turn the gain way down which actually lowers distortion
Now on the flip side you can save some money and get a 4ch amp that is stable at 2 ohms (most if not all are) and just wire your door and rear speakers together in parallel. You will loose the fader with this setup though. Finally you can get a 4ch amp and just use it to run the front doors and the sub and use your decks rear speaker output to run the rears. This way you get your fader back. Generally you want your rear speakers lower (quieter) than the fronts but that only if your really really into sound. Most people don't care about that. Hope this helps.