Well that and i've heard it all too. I've got 12s. Rock sounds awful, some rap sounds okay. Buddy of mine Noah had 10s in his GPGT, rock sound tons better, rap was blah, hip hop was okayish. Another friend of mine Brody has a single 15 ported ghetto style in his GPSE and rap songs on his kick ass compared to mine, rock sounds even worse than mine, and some of the more midrange and higher stuff in some pop and hip hop songs kinda sucked, kinda.
If a 10 is smaller, it can move faster, and have a lower xmax, and so songs with shorter quicker beats will be reproduced precise and tight. Then moving on up the scale applying similar principles to other sizes. 15s are bigger, heavier, will move slower, longer further excursion for longer lower beats.
Granted I could make rock sound good in mine by using a sealed box.
