Just stumbled across this thread by accident and thought I would add my 2 cents.
I recently swapped my roccat kaves for the CM storm sirus 5.1 because my kaves had broken.
I have paired both headsets with an Asus Xonar sound card which I bought originally because I wanted to have a set of 5.1 speakers connected to my on-board sound.
I had the roccats for two years, they did the job, but they were big, heavy and actually quite uncomfortable, which I just kind of accepted, but became more obvious when compared to the CM storm Sirus. They were quite bassy and you could tell they had a vibration unit in them, which was quite nice in some games but sometimes it seemed there was a bit too much bass and vibration.
The positional audio seemed to work pretty well, untill one day I noticed the rear left audio had gone completely, it became pretty apparent how useful the directional audio was at this point because I started getting killed in games like natural selection 2 because I couldn't hear the aliens sneaking up on me from behind as well because of my audio dead spot.
Also after two years usage the roccats had suffered a broken hinge on the left cup, which was held together by super glue! They are built from good quality materials, but the design of the way the cups rotate about their axis in both directions was just asking for trouble.
Anyway I replaced them with the CM storm Sirus 5.1, and decided not to use the USB puck and plug strait into my Asus Xonar. Initially I was annoyed with the frequency response of the headset and it took a bit of time playing about in the equalizer to get them sounding how I wanted. Out the box they seemed even more bass heavy than the roccats, however, they were just loud without all the extra vibration of the roccats. I felt they needed a tweek to boost the mid frequencies and drop the top/bottom frequencies a to sound how I wanted.
I'm no audiophile but they both sounded OK to me, bear in mind these are ONLY used for gaming so I'm not going to pass judgement on music/movies.
The positional audio was pretty good on both, but I would say it seemed a little better on the roccats, although I may adjust to the Sirus over time.
The CM storm Sirus is much, much more comfortable, they are lighter, they have less adjustment points and no silly rotating bits which can get broken, and yet they fit so much better than the roccats whilst feeling more solidly made. There were times I would take off the roccats and feel relieved that I had taken the headset off, which just dosen't happen with the Sirus.
Anyway this is all subjective and I'm awaiting getting blasted for not buying stereo headphones
