I've been in the market for a DAC/AMP combo around $500, and from many, many hours of reading, these are my impressions:
If you don't have a DAC, Yulong D100, NuForce HDP, and Audio-GD FUN seem like the best bets for combo units near $500.
If you already have a DAC, less expensive AMPS would be Schiit Asgard (Solid), Valhalla (Tube), and Matrix M-Stage (Solid). If you want to just spend $500 on just an amp alone, why not wait it out and get that Lyr that is coming out March ($450) that is a hybrid Solid and Tube, although it's probably overkill for something like the HD600.
If you don't have a DAC and don't want to go with combo units, I would split the purchase on a separate DAC (e.g., Mini-I, Matrix Cube, or NFB-12) and separate AMP (Asgard, Valhalla, M-Stage).