A lot of hi-fi headphones are much harder to drive, either because their input impedance is higher or they have low sensitivity. Whatever the reason, a headphone amplifier allows you to get the most out of them, whether it be volume or just allowing the headphones to perform at their best.
You can still have a 7.1 speaker system connected to the STX, you just use optical out instead of the separate outputs. Obviously, if the speaker system you want doesn't have an optical input then the STX is not for you and you'd be better off with the Phoebus.