Vortex Sound Research

I'm currently using some headphones that were thrown in for free with when I bought a used MicroKorg. They've actually been useful cos a) I don't disturb the family when I'm doing things at night and b) they're giving me a better idea of what's happening soundwise than my set-up, which is basically the mixer going into a home stereo.
I know they're not the best though so I want to upgrade. I don't really have a price-point in mind but I could probably go to around $100 and hope to find some good used ones. No idea where to start though, so, any recommendations? Basically want a very neutral response so I can really hear what's happening. Thanks!