Let me toss in my favorite headphone model: the PSB M4U2. This is a $400 pair of closed-back 'phones with a built-in amp and active noise cancellation. Don't buy these for the ANC -- buy them for the sound: clear and smooth, with clean highs, good imaging, and extended but not overemphasized bass. Very musical. There's a less expensive M4U1 model with no amp or ANC; these might be a better deal.

I was hoping the ANC would be useful in the office; it is, a little, but not hugely. Mostly it filters out low-frequency noise from the ventilation system, which didn't bother me anyway; it doesn't do so well with voices. But, it's pretty easy to mask voices with music.

Another recommendation you probably haven't heard: the German Maestro GMP 8.300D. I spotted a pair of these on eBay for $80, took a chance on them, and was pleasantly surprised. From 1kHz up they're stunning, with even better imaging than the PSBs, but they have a hump in the midbass response that annoys me a little.