Thought I'd share my experience.
I own the GSX 1200 and have been using it for a variety of games for the past couple of months, especially CSGO.
**Pros:** After owning it for some time, I've concluded the only reason to buy the GSX 1000/1200 is for its outstanding 7.1 HRTF DSP. It has the most natural sounding HRTF DSP available, and some people claim its 7.1 surround is more convincing than their actual 7 loudspeakers they've set up in surround. If 7.1 surround is important in your gaming (FPS) or entertainment (DVDs), this very expensive unit might be worth purchasing for you.
**Cons:**
Outside of its 7.1 HRTF, the unit is mediocre as an external DAC and underwhelming as a headphone AMP. The GSX 1000/1200 uses the Conexant CX20745 as DAC/AMP. Sound quality overall is good, but when I A/B tested against small USB DACs like the DFR (Sabre) or UD125 (AKM Velvet Sound), the GSX sound is grainier and with instruments, especially those in the low treble, more muddled and less separated. From a power standpoint, it will easily drive your HD598 to satisfying volume levels, but if you have any other Hi-Fi headphones, I can't recommend pairing them with this unit if you want to bring the best out of them.
Speaking of clarity and separation, the unit's stereo output is less than ideal when the unit is configured as a 7.1 device in Windows. So, switching from 7.1 (for gaming) to 2.1 (for most things else), in addition to tapping the indicator on the device itself, one also needs to go into Playback Devices in Windows and configure the GSX back to a 2.1 device. Going back to gaming would require the opposite steps. Someone here at head-fi made handy batch files (
https://www.head-fi.org/threads/sennheiser-gsx-1000-1200-impressions.824923/page-20#post-13379760) to do this for him.
**Summary:**
The GSX is a very expensive unit, and what you're paying for is first and foremost its excellent 7.1 HRTF DSP, and secondarily its convenient design. If I had to make the decision again, I would first test Razer Surround before deciding. As an avid FPS gamer with a competitive bent, 7.1 is very important to me. Overall, I do not regret my purchase, but I recognize the limitations of the unit.