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.

Checked the resistance across all 4 voice coils, i get 1.0 ohms (which is consistent with the reading I got when they were brand new unused). Does this mean the voice coils arent blown ?

[orange}
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{/orange]

Will give those ideas a try. Thanks.