I have been using a pair of second-hand Sennheiser 598s with Brainwavz angled earpads for about 2 years and they were a huge improvement over the SteelSeries Siberia v2 I was using before them. I only use these headphones on my computer with a Auzen X-Meridian 2G sound card. (Just a little side note, I bought this soundcard about 4 months before the company disappeared for about $180, and the best thing about it is the swappable op-amps on the board including a dedicated headphone amp. Still going strong after almost 8 years.)
Anyway, I listen to almost all genres of music on a daily basis via my 598's and really enjoyed their sound signature, especially with the Brainwavz pads. They really boosted the bass and have probably made the headphones "bass-head" phones, which I really enjoy.
I have been listening to the HD58X's now for about a week and wanted to write down my thoughts on them and compare to my modded 598's.
*Much clearer vocals. Switching quickly between the 2 headphones while listening to pop music, the HD58X has a clearer vocal sound that is almost harsh to my ears in the 1khz and 4khz bands. The HD598 almost has an "autotune" kind of sound on the vocals that I never noticed until listening to the HD58X's.
*Improved clamping force. This is probably more of a factor of having a used pair of HD598's, but the HD58X has a very high clamp pressure which actually keeps them on my head when I tilt my head down or bob along to the beat.
*Replaceable headband and compatibility with HD600 parts. The headband on my HD598 was replaced with a very cheap leather band from Amazon, which has actually held up pretty well and added a good amount of comfort to them, but was not officially for those headphones. The HD58X are compatible with all HD600 stock and aftermarket parts meaning their is a much wider range of supported products for them, plus it is much easier to replace the headband when it eventually gets dirty or worn out.
*Less ear fatigue at high volumes. I enjoy my music loud and usually for a few hours at a time. So far it seems that I can listen to music at higher volumes easier and for longer on the HD58X, as well as being able to listen to higher volumes (actual db volume and not windows based volume %) with less ear fatigue.
*Clamping pressure. Although I like having more clamping pressure to keep the headphones on my head, it is quite strong out of the box and was slightly uncomfortable. I slightly bent the metal headband strapping to reduce the pressure and it is better, but still not as comfortable as my HD598. This could also be due to having the harder memory foam stock earpads on the HD58X and much softer leather pads on the HD598.
*Less bass. Again, this may be partially the fault of the stock earpads, I am planning on buying Dekoni sheepskin pads, but the low end "boomy" bass of my HD598 is missing from the HD58X. Boosting the 30hz band results in a LOT of "rumble" but quickly distorts above 3db boost. Similar story on the 60hz band, but does not distort as badly and adds more of the boom I am looking for. Boosting the 120hz band adds a good amount of mid-bass but quickly changes the sound signature above 2db boost without a lot of extra "boom".
*About half the volume of HD598. Makes sense as the HD598 is only 50ohm and HD58X is 150ohm.
*Odd soundstage for some songs. I have been noticing odd soundstages for songs such as the center of the stage being either slightly left or right of center in my headspace as well as some songs showing left-right separation of instruments/vocals where my HD598s have instruments/vocals mixed mostly evenly left-right. I don't think I like this as it makes me feel like my ears are getting different sounds/sound levels which I have a small bit of OCD about.
*Short cable. I know that the shortened cable was part of the special run for these headphones since most people only wanted a short cable to go from their DACs, but I would have liked a longer cable as I run the cable almost 6 feet to have a comfortable amount of slack. I will probably extend the 3.5mm end of the cable.
Overall I have had a great listening experience with these headphones so far and I can see why they are being considered the best headphones under $300+. I am hoping that once I put some leather earpads on these that some of the missing bass boom will come back and the comfort will improve around my ears more. For the price I would say that anyone looking for something less the HD650 and higher should look no further than these headphones. The price is incredibly good for the sound, comfort, drive-ability, and compatibly with HD600 parts.