Having a set of HD6XX, these don't really offer too many surprises. I do like the more premium feel and look of the HD660s.
Sound wise, they are brighter than the more laid back HD6XXs but these do add more energy to the music that I listen mainly to - classical and jazz. As for bass, sure not big and boomy but its definitely there. I can compensate for more bass with EQ if needed.
Chuey1011Bass slightly lacking but very detailed and tight compared to the 6xx.
However using a compatible amp such as the Class A Rebel amp greatly increases the bass and overall richness of the 660S. Amazing pairing.
Update..
Using this via balanced output with the singxer SA1 is in a whole new catagory. Now my top standard combo. Sonically amazing.