Pros: Airy soundstage, sparkly treble, slightly lush mids, neutral bass, comfort, great looking, serious bang for the buck.
Cons: Mild upper mids coloration preventing them from sounding totally natural.
The HD 598 is a terrific mid-fi headphone. They sound very natural and almost effortless. They are extremely comfortable headphones. Also, they look like a luxury product.
These are excellent for gaming and watching movies. Also, these don't have high impedance like the higher end Sennheiser headphones. You can drive them with your iPod. Though, an amp can help you drive them to their full potential.
If you are a basshead , then stay away from them. It doesn't mean they lack bass, they have a perfect amount of bass for most music genres, though, it's not overly emphasized like in bass heavy headphones.
Talking about its sound -
They sound smooth with mild coloration in the upper mids. Apart from that - they sound natural.
Lows - well, they do bass, but it's not heavy hitting . There's an emphasis on the mid bass.
You can hear the sub bass, but it's mildly masked because of the mid bass emphasis. I wouldn't call the bass lacking . It's just neutral. Nothing special to write about.
Mids - Strongest feature of the headphone. It is something very different. I have never heard such a spectacular midrange before. It is lush sounding with a touch of brightness that prevents it from sounding too dark and laidback( it is still laid back though). It has an emphasis on the lower mids giving lushness to the sound.
There's a mild coloration in the upper mids . Upper mids are rather forward revealing harsh edges in some songs, though, It's not bothersome as they become smoother with burn in. The mids are nicely textured. Overall, HD 598's midrange is smooth and moderately bright sounding. Very addictive.
Highs and Soundstage - Highs are really good. Detailed but smooth. They prevent the headphone from sounding too dark. There's a mild recessesion at 8khz. though, there's a peak rising at 20 khz giving the treble a nice sparkle and airy sound. There's absolutely no sibilance to speak of. The treble is very nicely controlled, it blends perfectly with the amazing midrange producing a natural sound. This is furthermore helped with a very wide and deep soundstage. It is one of the best soundstages I have ever heard in headphones. It is not ultra wide like the K701 that can screw up imaging. It is decently wide and airy without feeling disconnected from music . Also, the depth is suprisingly well done.
Talking about instrument separation - instruments are separated nicely , you can hear each instrument clearly . Instruments have nice air around them.
All in all, if you are after natural sound and have a $200 budget, then buy them in a heartbeat.