In my experience the HD600 will not be good for for rock and metal. Let me re-phrase that: it will sound good but it won't impress you. Increasing the volume will not make the tremble sound more upfront it will still sound laid back. Do you listen to a lot of different genres as well? If not I would look into some AKGs (Q701 sounded extremely good with rock for me but made some genres sound horrible) Where as the HD600 makes every genre sound good but makes nothing sound amazing.
I listen to mainly rock and metal, but I listen to some pop as well. Do you think the HE400 will be a better option? People say it sounds really impressive at louder volumes.