Sony is a very popular company. $20 Sony headphones from Walmart sounded better to me than many ~$100 headphones. I'm listening to the Sony MDR-7506 right now. It's one of the most popular studio monitor headphones of all time. (Not sure why, because it has some nasty treble peaks and such, but EQ can fix that.)
the ones you own, mdr-7506 are known the be very good studio monitors.
the xb600 are more or less bass cannons, which are not as clear sounding.
YKYAAW you use
HQPlayer.
Guys...seriously...download the free 30 day trial of this player ASAP and listen to it with all your music. It sounds so much better than every other player I've used (JPLAY, foobar2000, iTunes, Winamp, etc.) it's insane. This is the only player I've found that makes an immediately and hugely obvious improvement in sound quality that is genuine beyond doubt. Sounds so urgent, exciting, and dynamic, it's almost like upgrading to better headphones!
i have nothing against DSP processing, but do you know what kind of dsp processing is donw within hqplayer? software speaking, it should sound the same as foobar and others, if it does not it does dsp, with which i agree, but i am afraid that i might like how it sounds, and might want to keep it after trial ends.
EDIT2:::
tested hq player. it sounds interesting. it does heavy dsp processing, but sounds really good. i still cannot put my finger on what dsp it does.
EDIT3:::
amazing music player... this makes me wonder, what music player sounds the best? this one works very very slow, and the resampling algorytmhs are hard on both processors, and the system, and i cannot simply play something else, it takes control of audio output.
this is why i liked foobar, it was simpler, but as this sounds better, i might want to test it deeper.