A little more detail on my headphone situation:
I like the Senn HD280s because they're closed enough to be good for tracking, but still comfortable (for me) and good sounding (to me). They also expose a lot of detail, so they're perfect for picking through tracks late at night to edit out crap like microphone bumps and stuff without waking anybody up.
I agree they're not flat -- I've tried doing mixes with them, and it never works (OK it never *works* for me to do mixes with headphones at all, but I'm under the same space/time/noise constraints as a lot of HR folks, so I still try to do it a lot, but ultimately break out the monitors, and my mixes with the 280s are more likely to be *way* off than with another set of cans I've got:
I've also got the AKG K240 headphones - I didn't know they were discontinued - that sucks. I think the response is flatter, but they're open, which for me makes them a lot less useful - that is, my wife/kids are almost as irritated with me repeatedly mixing with the K240s as they are with me mixing at low volume on my monitors, and the results are a lot better when I use the monitors (admittedly I still suck no matter what when you start comparing me to other people, but that's not what I'm getting at

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BTW, on the subject of mixing with headphones, my experience abundantly confirms the information in this article:
http://www.bluebearsound.com/articles/headphones.htm (and it looks like the mannequin there is wearing K240's

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edit: but to the article, I would add, "so what?" sort of -- if I were a professional and/or charging other people money for my work, things would be different, but *I'm not* - if my mixes eventually sound good on my computer/my phone/my van/my car/my headphones/my buddies' computers/etc., then I'm good, and I've got months/years to pick at them, and no pressure to get things right the first time.