Have you checked the gain on the amp?
Is the sub sealed properly in it's enclosure?
Is the enclosure the right size for the sub?
Have you got the sub wired out of phase i.e. positive and negative terminals round the wrong way?
I cleaned the EGR on my Avensis Diesel.
Removed the EGR from car, brushed out what carbon I could and then cleaned the rest out with Carb/ Injector Cleaner and turps.
Think I'd use my ultrasonic cleaner now if I'd have to do it again.