I've used the DSS for a few games now, so for anyone who's interested here are my thoughts:
The first thing I noticed when I plugged in my DSS was that the sound coming through the headphones was much more clearer and crisper. The bass was also noticeably punchier and just plain awesome.

And the bass dial on the DSS wasn't even close to its max!
Also, even though the volume was loud, my ears did not feel any strain or pain. Under normal circumstances (stereo sound), this volume level would have been unbearable. I guess this is a byproduct of the surround sound effect. It is as if the sounds are emanating from around your head instead of coming from inside your head.
In RDR, when I was riding my horse around in the wilderness and heard a gunshot in the distance, I could pinpoint the sound's direction and distance accurately. And oh boy, all the ambient noises such as my legs brushing past a bush, a dust ball rolling down a hill, and the creaks of a carriage while it rolled across a rail track could be distinctively heard. I guess the DSS "expands" all the sounds and puts more weight in them. For example, the thunder sounded very bassy and I could it seemed as if everything around me vibrated. Without the DSS, the thunder would have sounded static and somewhat flat in my headphones. Same with all the gunshot sounds.
And yes, when someone fires a gun at you, it does feel as if a bullet just whizzed past your head!