The Astro A40's only have two physical drivers in the cans
, one in each cup of the headset. If you do end up going for the optical input you're only going to get a virtual upmixing since there really isn't a 7.1 driver setup in the two ear cups of the headset. If you do want true surround, you should look into the Razer Tiamat Headset which works off of the analog 7.1 connections on your soundcard(dedicated or onboard).
You can look into Asus' Xonar DGX but I think you should read through this