Oh Jesus, here comes the "omg liek 18s cant play rock music cuz thair 18s cuz it so big lolz" garbage. It's not the size but the design of the sub that's really limiting it's upper frequency extension. Also, PESteele, you may be having suspension issues. While playing the sub free-air at low volumes, run some tones through it and see how it sounds. Try with the low pass filter on and off. Try from about 60hz and play tones up to about 100hz. If I recall correctly, the Qs have dual spiders and one of them may have become separated and it might be the cause of your bad sound.
Everybody else may just have an issue with terrible midbass from their front stage. Can't you people run some test tones and see where output is lacking? Saying the "upper note on song 'x'" is a terrible way of identifying a frequency when there are tons of tones on the net to download.