It's really preference.
Some people watch movies and like 5.1 setups.
If you add digital sound, sometimes that can make the sound clearer. The source and where it amplifies it stays constant since the signal is digitized and then decoded near the amps.
I too started with a headunit with aux in. And ran my soundcard to it. I later got a USB soundcard, then another USB soundcard with 5.1 and connected that to my amps without a Headunit.
Now I got a 5.1 DSP processor and run optical into that and that routes to my amps.
What do you like about the 6.5v preamp outs so much?
Your MII12K has a Spdif coaxial out, so you can hook that up to a DSP processor like mine and ditch your headunit if you want.