"Schizoid Man" said:

What exactly is the difference in the input stages between Phono and Aux for
regular stereo receivers?

Yep. Aux will accept any line signal, like a tuner, CD player or
tapedeck.
Phono will accept only tiny signals from a turntable cartridge.
It is designed to handle signals in the 1...5 mV range and has a
so-called RIAA equalization.
So the phono input can't be used to connect a line signal to.

Note: you can build a little circuit like given in the link below to
convert a line signal in an acceptable format for your phono input.
The text is in Dutch, but all you really need is the schematic:

http://www.circuitsonline.net/circuits/view/86

Will I damage my speakers if I connect an Xbox to the phono inputs? I only
have one Aux and that is being used by the cable box.

If you set the volume at normal listening levels, the sound will be
extremely loud and full of bass. Not recommended.

--
Sander de Waal
" SOA of a KT88? Sufficient. "