headphones for mixdown/mastering
I recommend the AKG K240S without hesitation.
I went through this exercise early in the year and spent weeks doing my homework. I ended up, happily, with the AKG K240S. They're extremely comfortable and they have a much more accurate portrayal of the low frequencies than the Sony 7506 (which led to final mixes that were too lean in the bass, given their accentuated low end.) I sent the Sony's back. The Senn's were not as comfortable as AKGs. The Audio Technicas are too boomy in the bass. The only one I did not have a chance to try, but it's a bit more expensive, is the K240DF. It's supposed to be the most "neutral."
By the way, some like the AKG K240M for mixing or mastering. I use this for tracking out of the mixer's headphone output. But for the overall mix, it's impedance is too high for use with standard headphone outputs of components (requires a mixer's headphone out or a separate headphone amp). I like to hear what's going directly onto the tape, so I use the tape machine's headphone output. Also, the K240S is a bit more revealing overall of detail. I'm happy with my mixes.
There's always the possibility that style of music plays a role in all of this. . . I'm doing jazz and ambient, not rock.
Good luck.
Michael