Have you tried running "alsamixer" in a terminal and then lowering the volume (using the arrow keys) on all capture/recording channels like microphone/CD/line-in (you may have to press tab once or twice in the interface to go through them all). That is a common cause of hiss on many sound cards.
Have you tried running "alsamixer" in a terminal and then lowering the volume (using the arrow keys) on all capture/recording channels like microphone/ CD/line- in (you may have to press tab once or twice in the interface to go through them all). That is a common cause of hiss on many sound cards.