If that were even possible, using the 3.5 out of the PC would make having that amp pointless especially since it has digital inputs. If I remember correctly, I looked into getting this amp when it was new, and as it turns out there is only one way to use that amp and that is if you have a compatible alpine headunit .
When I looked into this amp before, I "thought" you could use the RUX-C701 as the controller for stand alone use to control volume and other sound adjustments just like it does with the PXA-H701 processor. Then all you would need from your PC is an optical output. But as it turns out that it is not possible especially since it uses different plugs for the controller, and that amp doesn't even have a remote turn on port so you couldn't even turn it on.
So, it's sole purpose is to use it with a specific series of headunits.
I ended up going with the PXA-H701 instead and using different amps.
I'm not saying that saying you should go non-alpine for the amps, Hell I own 2 of their PDX amps myself. I just saying that you should stay away from that one if you are doing a computer without a headunit.