I use Sennheiser HD280s and really enjoy them. Good isolation and good frequency response. The low end feels a LITTLE exagerated to me due to the closed back design and tight fit. Also, they can be a little uncomfortable after awhile, 2 hours or more, but if you are mixing, I would suggest taking a 10 to 15 minute break every hour or so to refresh your ears, so that can help the comfort issue.
My roomie had the Sony 7506s. Really flat frequency response too, but not as isolated as the HD280s. They are really comfy too.
A friend of mine had the Beyerdynamic DT770s. These sounded great! Better than the Sony and Sennheiser headphones. VERY comfortable too, but again, not as isolated.
Also, a few other friends of mine have the AKG K141s. I haven't had much listen to them, but they really liked the comfort and frequency response. They said it had a more natural feel because of the open back design, which may not resonate and exagerate certain frequencies because it doesn't have the closed chamber the closed back headphones have. But the downside is sound comes in and out. So if you are going to be in a studio alone without too much outside noise, these might be better for you.
Another friend of mine has the HD600 or HD650s by Sennheiser, open back headphones, and really likes those. Swears by them. If you have the money invest in a
nice pair of headphones.
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