Good to know, nickchen. I've always preferred flat pads on my Grados.
Zuqi, it takes a while for Grado headphones to burn-in. I listen to a variety of music at regular volumes - I'm not a fan of the more exotic burn-in methods (e.g. pink noise, bassy music, loud volumes, unattended weeks of playback, etc.). It's actually kind of fun to hear the differences in the headphone's output as it changes over time - Grados, like wine, improve as they age. Sometimes the changes are gradual, sometimes they are a sudden revelation.
My headphones sounded really unimpressive at first - really flat and uninvolving. It took a few months for them to settle into their current sound signature - I've been listening to them since 2004. Also, the great thing about headphones is that you can easily take them places - try them in different systems if you can.
I suggest that you enjoy your new cans for a few weeks before making any decisions about their sound quality. Also, the bowl pads tend to be a bit stiff and scratchy when they are brand new - they will soften over time. Like everything else in life, headphone listening is a journey, not a destination.