I recently had a new subwoofer installed in my 2007 Honda Civic Coupe EX. I also have a new Kenwood HU. After the sub was installed, I noticed what seemed to be a ground hum every time I turned on the car and released my parking brake. The hum would continue as long as the engine was running, but not when it was off. Anyway, I took it back to the Car Stereo place and they said it needed a Noise Filter to remove the ground hum. They installed this and it did get rid of the hum, but it also seems to have made the subwoofer almost gone. I can barely hear it unless I crank it up all the way, and even then it's not great. Before the filter, I used to be able to keep it at +1 and be happy; now I have to keep it at least +6 and often to +10. Am I going crazy or is the filter attenuating the subwoofer sound?