7.1 Setup for ASUS P9X79 Pro

I have the same motherboard. I was fortunate enough to buy a CreativeLabs T7700 7.1 speaker set back in the day and use the analog outs with that. I have since looked for newer 7.1 PC speaker sets but have yet to find another analog set like that. No one makes more than a 5.1 system these days. In these speaker sets, usually all speakers and the sub are powered by a power amp in the sub.

You could also look for a used or cheaper 7.1 surround amp/speaker set. These usually have one s/pdif or coax input at most. Look for one with a s/pdif connection. You could just run a s/pdif cable from the motherboard to the surround amp, so you have another option.

Your other option is like you said. Get a good 5.1 setup and then for the side speakers, you could purchase a separate two-speaker set and plug them into the gray output on the motherboard. You could still control the volume of both through the OS, but you'd have to set the master volume on both the 5.1 and the 2-speaket sets at some max level where the volume/balance between the two is close.