I auditioned the 7506/V6 briefly. It was competent but not stunning. I can't say I found the bass lacking in quantity, and the extension was pretty decent. But it definitely lacked control. The upper mids were quite present, more so than the other two headphones I tried (M50 and SRH440), and I honestly can't remember much about the treble. The thing that struck me the most, though, was the lack of transparency. They were grainy compared to the other two--I never quite understood that term until I heard those headphones. I voted "decent."
To the OP: You might consider this turn of events a blessing in disguise, though it's definitely a bummer. If you're not stuck on the V6, you might consider some of the popular (or deserving of popularity) choices mentioned on here like the M50, SRH440, D1001, HD448, etc. All are a little more expensive than the V6--especially if you've got a fake one at a discount--but you might find one of them to your liking. The V6 is a polarizing headphone--you've seen that in the responses. It's also been around since the time dinosaurs were stomping around, and other makers and models may have surpassed its value.