So i've been trying out the GSP 670 and looks like Lord Vile was right:
I have to say I'm really dissapointed that even such a "high-end" wireless headset (which is praised for it's sound quality by many reviewers!) doesn't even come close to my DT990's. The audio quality is mostly just "ok" for me. And the mic is just atroicious, even a 10€ Zalman ZM-Mic-1 sounds better. I'll send the sennheiser headset back. Other than that i got SEVERE interference issues as soon as i stood up from my desk or even turn my head away. Overall i can't recommend this headset at all. Looks I'll end up with the wired DT990's and the Modmic because there are just too many downsights. Well, it was worth a try, if only just to know wireless headsets or just headsets overall are just not up to my standards. Not even "high-end" ones for 350€.
For sure i was prepared to sacrafice SOME quality for the comfort of wireless, but I didn't expect the wireless headset experience would be that bad at all...
Thanks anyway guys!