AKG K701s are about as nuetral as you'll get. My opinions of the other hi end headphones:
Ultrasones: Proline 750s and 2500s: Can't believe that the ones I heard weren't broken. Hands down the worst sounding transducers I've ever come across.
Sennheisers: HD580s, 600s, 650s: Very pleasing to listen to. Definitely a tonal balance that leans towards the warm side. More laid back sound. Not particularly nuetral, but very enjoyable, relaxed listen. Probably my favorite can for casual listening.
, 831, 990: Brighter than the Senns, with a bigger bottom end as well. Fun to listen to, and pretty clear/detailed/accurate/etc. Probably my second favorite can for work.
Grados: Entire Line: Too fatiguing for me. They are bright, punchy, and in your face. I feel like I'm getting attacked when I'm wearing a pair. They're definitely detailed and smooth, but I can't stand them. They sound good, they're just not to my taste. Lots of people love them though, which goes to show that audio is all subjective.
AKGs: K601 701: Most nuetral cans I've ever heard. Very clear, detailed, clean. What you hear is what's there. These are the Dynaudios of the headphone world. If you're looking for a set of cans for casual listening, you'l lbe better served elsewhere, but for work and analyzing mixes, these things rock. Plus they are very comfortable. Highly reccomended.