I had problems with my 8" kicker solobaric... the dust cap had worked itself loose. I just carefully peeled it all the way off and then glued it back on with a light layer if glue.

Another thing to check would be to see if one of the voice coils is blown. No sub should give dumpy performance above 50 Hz unless it's over 24", and even then I'm not so sure.

It COULD be the amp, but I doubt it. Check the amp outputs with another speaker to rule it out... then try and check the speakers individually. One blown voice coil will probably make the whole sub sound like crap.