guardianx9If you like warmer sound go with hd6xx. If you like brighter sound K7xx. If you like vocals then 6xx. If you like pop, rock etc then K7xx as you might like the soundstage better. If you like a headphone that screams stealth then 6xx; if you want to look like an alien who just jumped off his spaceship then k7xx.
Note: you can mod 6xx easily and the sound becomes even more refined. I haven't done bass mod for my K7xx, as they have enough bass for me. I prefer 6xx to k7xx because i like warm tilted sound (esp. after a few mods which reduces the amount of warmth) and it's unbelievably smooth for vocals and highs are non fatiguing. YMMV.