I don't own the headphones, but I've read a lot about them (because I'm also looking for a headset for the same reason like you). The HD598 is a headphone with excellent soundstage, but lacks a bit of bass according to reviews (I've tested it instore, it seems fine to me. But everyone his/her own opinion ofcourse).
In the MLE Headphones gaming guide topic it get's the following score:
Fun: 6.5/10 (Pretty Decent)
Competitive: 9.25/10 (Amazing)
Comfort: 7.25/10 (Good)
The review:
You all know it as an improved HD595, which is a consistent fave among gamers, especially those who use virtual surround amps like the Mixamp. My opinion? The hype is well justified, at least when it comes to the HD598. They are incredibly well balanced. Somewhat bass neutral, mids are pretty up front and center, and neutral treble (never harsh) translates to a very promising headphone for gaming on paper. And I'm happy to report that they are ABSOLUTELY phenomenal for gaming. They belong right up there with the best for competitive gaming. Soundstage? Check. Positional accuracy? Double check. Detail retrieval? Triple check. The trifecta. It's missing very little for those needs. The bass may be a bit too laid back for those wanting some immersion in their non-competitive gaming however.
Comfort-wise: It's a hit or miss. The pads are huge and give your ears plenty of space to breathe. My issue is mostly with the clamp. Sennheiser just seems to design some pretty clampy headphones. Like the PC360, the only way I can find them comfy is to over extend them, and give them a loose fit. The headband padding is full on pleather, which is less comfortable than the PC360's velour padding.
Apperently, you can mod the HD558 so it's almost equal to the sound that the HD598 provides.
Both headphones are open, for the better positional accuracy. If you take a closed headphone you will mostly lose that, but will get more booming feedback/feeling.
If Sennheiser is the only branch you can get, this is what I've got after a lot of research.
Otherwise the Beyerdynamic DT990 Pro will probably give you more fun. Less positional accuracy (but more then a closed headphone) and excellent booming feedback/feeling. In the MLE topic I've mentioned earlier it gets the following score:
Fun: 8.5/10 (Excellent)
Competitive: 8/10 (Great)
Comfort: 8.5/10 (8/10 for the 990 Pros due to stronger clamp)
I myself ended ordering the AKG K612 Pro, which got the following score in the MLE topic:
Fun: 7.25/10 (Good)
Competitive: 9/10 (Amazing)
Comfort: 7.75/10 (Very Good)
Overall: 8/10 (Great)
In the end it's all a matter of opinion. Like I said, I've tested the HD598 and I thought the bass was more then fine, but you may think otherwise about it. It's very difficult to order/buy a headphone blind, without testing it. Altho I did that with the AKG K612 (couldn't test it out, but because the AKG K612 gets overall better reviews then the HD598, and is 40 euros cheaper in my country, I took the ''risk'').