Do i need a processor?

i have a sa-ax6 receiver that i am very happy with, and a sh-ac500d processor

im using this mainly for my computer but i am having trouble connecting it.
originally i was using 3 aux to rca cables, but this receiver is from before built in processors and i had heard that having one could greatly improve my audio quality.

however after connecting the rca's to the processor i discovered that the processor sets itself to "through" and has no effect on it,
so than i tried fiber optics, but i cant seems to get my computer to play audio through dts.

that being said is it worth it to try and figure out how to get dts to work or should i just go back to the rca setup?

i was also looking at using the fiber optic out on my tv, but i can only seem to set my hdmi to 2 Chanel, meaning i would loose my surround

my initial setup was useing 3 sets of aux to rca cables for 5.1 surround sound, im just curiouse how important using a processor is, and how much quality difference i will hear