I advise people against the HD6XX / 600 / 650 headphones. They are solid mid-fi performers from a technical perspective but seriously lack euphonics so people will inevitably get tired of them and look elsewhere. If you are stuck on a mid-fi budget (ie, $300-$800) consider the HE-400S or TX-00. Nighthawks are also on sale for $350.
My main recommendation is to find the budget for hi-fi headphones like the TH-900, LCD-X, HE-X, Elear, etc.