Can't select supported max. resolution with HDMI Ubuntu 13.10 AMD card

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http://askubuntu.com – I've got an AMD graphics card (6900 series) and just installed the beta driver (after fighting with the stable one). I'm also using a screen with 2560x1440 resolution connected via HDMI which works fine on Windows (so I assume there is nothing wrong with the screen itself) but not on Ubuntu. The maximum resolution I can select is 1920x1080 which looks ugly. However, I can use 2560x1440 with DVI but I use the built in speakers via HDMI so I'd like to keep that. I've tried to use xrandr to basically fake it but xrandr can't find a HDMI output I could add the new mode to. My OS is freshly ins (HowTos)