My office setup consists of a Little Dot MKII (tube amp) powering a set of Sennheiser HD-650's. I'm pretty new to the headphone+amp scene, but I absolutely love this setup. I almost (almost
) perfer the sound over my home Axiom setup. Can't get the depth and 'spaciousness' of the home speaker system, but the overall quality of the sound is delightful. Creamy, buttery, sexy, & warm. Like biting into a perfectly marbled, aged, prepared, & cooked steak.
I've also demoed/borrowed a pair of AKG 701's for a couple of weeks from HeadRoom. They were truly excellent headphones as well. Just not quite as 'creamy' as the HD650's, but it really boils down to personal tastes.
On a more modest budget, I'd heartily recommend a pair of Grado SR-60's. For the $65 or whatever they are these days, they're a steal.
Oh, and Head-Fi is an evil
place. Truly a place where bank accounts and credit cards fear to tread.