Little late but here are my impressions of soundstage modded HD598s.
@Evshrug, sorry if my writing style isn't as good as MLE's or Jude's, I mostly deal in numbers, not words.
Let's start with bass:
Difference is not big, headphones didn't magically transform into some kind of Megabassozord but there is more of it. I noticed it immediately while watching latest episode of Sons of Anarchy. Both Jax's voice and the motorcycle engines sound fuller, the vibrations of both the voice and the engines are more pronounced.
Nick Cave's Where the Wild Roses Grow are now a different song to me. Previously I could barely hear Nick's voice in the chorus and it sounded more like somebody added unnecessary reverb to Kylie's voice. Now I can clearly hear him singing along. If 598's bass was like an acoustic guitar then the modded 598's bass would be the same guitar but with thicker strings. A subtle but noticeable difference.
The mids:
Mids weren't affected as much as the bass. The whole "thicker strings on the same guitar" impression remained but it's less noticeable. Intro to An Inifnite Regression by Animals as Leaders sounds pretty much the same, maybe a little bit clearer. Meshuggah's Dancers to a Discordant System were more affected by the mod. The main guitar riff sounds more like an actual instrument playing in the room and less like a recording played through headphones.
Overall, the instruments sound like somebody cranked the amps up to eleven while simultaneously keeping the volume on the same level. More sound in the same amount of decibels (I hope You guys get what I'm trying to say here)
The highs:
Either my ears are damaged from sitting in headphones all day or there is no audible difference . Fortunately they don't seem to sound any worse either.
Soundstage:
The whole reason for doing this mod. Well, to put it shortly the improvement is tremendous. I heard it best while listening to Nancy Sinatra's These Boots" The instruments are much more clearly separated now. The days of double bass blending with the guitar and the electric bass becoming inaudible after Nancy starts singing are gone.
As for the games, I launched the Silent Hill demo (which I still keep on my HDD mostly to show people who come over that virtual surround is not a gimmick).
First thing that stood up, the footsteps are now directly on the ground and not to the front of my character like they used to be. The clock makes a full circle around my head while rotating the camera instead of this weird elliptic shape. Unfortunately I had to stop the tests after about five minutes because I was getting real close to screaming like a little bitch.
Next game on the list: Battlefield 4. I opened up the Firing Range. As was the case with P.T the soundstage changed from a slightly elliptical shape to a full circle. Moreover my own gunshots were positioned in front of me so naturally that for a moment I thought the sound was actually coming from the monitor's inbuilt speakers. Explosions haven't changed much though.
I hope somebody will find this post useful. As for the mod itself it takes about 20-25 minutes for each cup. Keep in mind it's non-reversible.