Another DAC will not improve your Saturn, it will give you the sound of another DAC (and that of the associated cables required to connect it) !
If you want to seriously improve the sonics of the Saturn itself however,
(from direct experience with my own Uber Clocked Saturn: http://db.AudioAsylum.com/cgi/m.mpl?forum=digital&n=142994&highlight=terra+firma&r=), I can wholeheartedly recommend the Custom Analogue/Vacuum State electronics Terra Firma/Uber Clock.
http://vacuumstate.com/fileupload/NewsletterFeb_2009_1.pdf
If you want to seriously improve the sonics of the Saturn itself however,
(from direct experience with my own Uber Clocked Saturn: http://db.AudioAsylum.com/cgi/m.mpl?forum=digital&n=142994&highlight=terra+firma&r=), I can wholeheartedly recommend the Custom Analogue/Vacuum State electronics Terra Firma/Uber Clock.
http://vacuumstate.com/fileupload/NewsletterFeb_2009_1.pdf