There are lots of great choices within your price constraint. The Sennheiser 600 already recommended is an easy choice, and well worth the premium over the next most expensive Sennheiser. Both the Grado RS-1 and RS-2 are worth considering, especially if you are going to consider the Grado headphone amp (the cheaper RS-2 seems to me the better value). For a more exotic choice, the Koss electrostatics are something of a black sheep, tarred by the rest of the Koss line, but, if you care about sound more than the sexiness of the marque, they really deliver--in my opinion a much better value than the comparably priced Stax alternative.
