I have both and to my ears the consumer V6 sounds much, much smoother.
I have both and find neither "smooth." I retired the V6 after I got the
7506. I use a Sennheiser HD600 if I want a really good headphone sound.
But, all three hurt my ears after a short time of listening. I use the
V6 plugged into a computer, for the rare occassion where I'm interested
in hearing something coming from it (no sounds otherwise).