subsonic is 75% of your box tuning. For gain, go up until it stops getting louder than back it off a few notches. Then play your hardest hitting music for 5 mins then see if the sub or amp gets hot, pay attention to smells throughout the whole time. If either equipment is hot, back it down even more, let it cool off and do the test again. If its cold, you can raise it up a bit. If its room temperature then you are at a good spot.
Eventually you'll want a JL cleansweep or a kicker sum8 or a full blown processor like an audison bit 10 or bit one or helix etc... That line out convertor is holding you back several dbs with a sh*t stock head unit signal.