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.
Gaming
One major advantage of using an open-back gaming headset is that you inherently get a better soundstage than you ever could with one that sports a classic closed-back design. Wider soundstage means that the sound is larger, unrestrained and airy. When playing videogames, that translates into exceptional spatial awareness and immersion. These two aspects of the Sennheiser Game One's sound reproduction initially impressed me the most. I play a lot of PUBG (strictly FPS servers) and Battlefield 1, two multiplayer shooters where a winner of a duel is oftentimes decided by who fires first. Sennheier's headset constantly kept making sure that the player pulling the trigger first was me. I was able to hear every single sound in my vicinity, including the most subtle ones. The same was the case with distant gunfire and approaching vehicles. When you're that aware of your surroundings, the chances of someone getting a jump on you are slim. In short, if a competitive edge is what you're after, the Sennheiser Game One delivers in heaps.
It's not only good for that, though. Yes, the sound it produces is very precise and exceptionally detailed, but it's also warm and rich, with a refined, weighty bass and smooth highs. Thanks to that, you'll have a great time playing other types of games, such as those that heavily rely on atmosphere and dialogue. It's just a great-sounding headset regardless of what you plan to use it for.
Since this is an open-back headset, the passive noise isolation is practically nonexistent. You'll be able to hear everything happening in the room you're sitting in, and everyone around you will hear what you're listening to. Therefore, you really do need to have a dedicated gaming room to be able to properly use the Game One.
Music
Everything that makes the Game One great for playing games also makes it enjoyable for music listening. The width of the sound stage and sheer amount of detail in the mid- and high range translates into excellent instrumental separation. It's next to impossible to find a song or music genre that would choke this headset.
The bass is tuned in a way that gives it good depth, nice weight, and quite a bit of punch. At the same time, it isn't overdone, which is where serious audio companies like Sennheiser vastly differ from many gaming peripheral manufacturers who often can't resist going overboard with the low-end. That's because Sennheiser doesn't have to use the bass to hide problems in the mid-range. I really enjoyed how the bass has the thickness needed to properly interpret various ambient and rap songs and the snappiness necessary to successfully tackle genres like rock and metal.
Long story short, the Sennheiser Game One transforms from a great-sounding gaming headset into a lovely pair of hi-fi headphones seamlessly. Like any other good pair of hi-fi headphones, it benefits from a better DAC, so if you like what you hear when you plug it into the sound card that's integrated into your motherboard, good news - it can sound even better. If you'll be looking to grab a good external DAC to combine it with, check out the likes of the AudioQuest DragonFly Black, FiiO E10K, and Cambridge Audio DacMagic XS V2. As for external sound cards, good choices would be the Asus Xonar U5 and U7, Creative Sound Blaster Omni, and BlasterX G5, or Sennheiser's own GSX 1000 Audio Amplifier.