I'm using DAC+, and I have a problem doing what is written here : http://www.hifiberry.com/guides/hifiberry-software-configuration/
Indeed, after updating firmware (sudo rpi-update) and reboot, the system actually doesn't boot anymore : I have that classic rainbow startup screen, and then a second one with a small red square at the top-right of the screen. It stops there.
When I remove the DAC+, it's booting fine.
Not booting after updating firmware