There is no way that a 300 Ohm can in general and the HD 600 specifically (with a lower than 100 dB sensitivity rating) should sound the same unamped vs. properly powered.
Unamped do they sound like this?
http://www.head-fi.org/t/634201/battle-of-the-flagships-58-headphones-compared-update-audeze-lcd-2-revision-2-6-4-13#user_HD600
Unamped they sound the same as amped so, the Bass is very clean, has good punch, mids sound neutral and dont seem to have color and are also pretty detailed and clear, treble is very detailed depending on what tracks I listen too and music genre, treble is pretty airy and the headphones do sound over all transparent, there's no veil, muddiness or crap sound I'm aware of. Lets some it up, they sound balanced, clear, neutral and have decent amount of detailed. Yup both unamped and amped.