I'll split the audio performance analysis into two parts - gaming and music. Even though the headset produces the same sound regardless of what it's being used for, you won't necessarily look for the same things while annihilating your virtual opponents as you will when listening to your favorite tunes.
Even though it has numerous other qualities, the sound performance is what really sets the Sennheiser GSP 600 apart from pretty much any other gaming headset on the market, and what makes it well worth its price. The built-in speaker drivers have been tuned in a way that makes the GSP 600 sound fun, explosive and dramatic, while at the same time maintaining excellent levels of detail and clarity. The bass is deep and exceptionally rich. It drops low and hits hard, thus removing any chance that you'll find yourself running through virtual battlefields with an expression of boredom on your face. Explosions sound magnificent, gunfire is exciting and oftentimes heart-stopping, shouts of soldiers and voices of NPCs are clear and warm—it's a kind of a sound you'll easily fall in love with.
The GSP 600 also does wonders in terms of spatial positioning. If you're into multiplayer shooters, you already know that a good headset, one that makes it easier to pinpoint the exact location of your enemies, gives you a competitive edge. With its terrific spatial positioning, the Sennheiser's headset had me comfortably navigating my virtual surroundings because I soon realized that there's practically no way someone can approach me without being heard. All you need to do is listen—the GSP 600 will happily present you with the faintest of footsteps. Someone's shooting at you from a distance? No problem—this headset will save you from turning around in panic as you'll instantly know where the gunfire is coming from. As far as I'm concerned, the spatial positioning of the GSP 600 can be considered eSports-grade.
Thanks to the design of the ear cushions and closed-back nature of the ear cups, the passive noise isolation is great, and there's no sound leak to speak of.
A great pair of headphones will sound awesome regardless of what they're being used for. The Sennheiser GSP 600 confirms that wholeheartedly, as it seamlessly transforms from a great gaming headset into an excellent pair of headphones for your music listening sessions. If you don't already own a pair of good Hi-fi headphones, you will essentially kill two birds with one stone by buying the GSP 600.
The aforementioned weighty, punchy bass, combined with a detailed mid-range and clean highs, does wonders for various music genres. The transition between the bass and mid-range is admirably smooth. There's no annoying 80-100 Hz boost that would make guitar-driven genres sound too bassy and annoying. The vocals are pushed forward a bit and can at times sound a bit shouty, but their excellent clarity leaves little to be desired. The width of the soundstage isn't exceptional, which comes as no surprise given the closed-back nature of the headset. A wide soundstage is something we'll look for on the open-back GSP 500. All in all, I don't think I've ever tested a gaming headset I found as enjoyable for music listening as the GSP 600. There are no music genres I'd consider it unsuitable for, and that's a feat on its own.
If you own a higher-quality DAC/headphone amp, do use it for the GSP 600 as it can and will benefit. Further improvements in tightness and overall balance of the sound were easily discernible after I switched from the integrated sound card to a solid USB DAC, such as the FiiO E10K and AudioQuest DragonFly Red. Even more so with the fantastic $800 Audiolab M-DAC.