I was on a similar search a few years ago, and I decided to go with EquationAudio's RP-21 headphones. After some listening tests using my own iPod and familiar songs, tones, and sweeps, I felt they offered a level of accuracy that I wouldn't find in a similar price range. They isolate well enough for line checks and occasional critical listening to dial-in settings.
I got mine for $80 from 8th Street Music. They let me demo them before I bought them…definitely a nice perk to have. I replaced my "venerable" Sony MDR-7506 cans, and whenever I listen to them now, they are very shrill to my ears.