I've always run a hu that controlled the sub separately, being able so adjust the volume to the sub alone is great. I have a pioneer 4900ib has hi pass x-overs for the speakers and not only separate sub volume, but it limits the frequencies to the sub as well. Bass blockers are a pain, with a good deck you'll improve your system 1000%. Also my sub fires to the rear of my hatch sounds better when the bass reverberates off the hatch door. I took the same sub, box and amp out of my truck and put it in the hatch and it was much louder. The gain is not a volume knob, it equalizes the signal from your deck to the amp. So for example if your deck has a 1 volt preout your gain would be turned higher than one with a 4 volt preout. Personally I never turn mine passed halfway. If you do decide to turn your gain up and your amp starts to get hot turn it back down, you'll destroy it. I've let the smoke out of 1 or 2 myself in my unwiser days. A nice system is when you can turn your bass to negative 15 on your deck and still knock off your rear view mirror.