Sennheiser HD 598 Audiophile Headphones with High-Gloss Burl Wood Accents

In a hurry so I will try to be quick, but thorough. Forgive the lack of proper structure but I think you will find the review quite informative.

I think it would be quite helpful to do a comparison. In contrast to the 598s, the HD555 are quite affordable to most for a beg head fi headphone, and they do sound good. Also, yes the foam mod "works" when done on the 555 in that it opens up the headphones, bass is more prominent but still remain tight (this is a popular mod that is well known in the head-fi community).
The 558 mod (foam removal) will NOT make the 558 like the 598. It will make the bass somewhat heavier/looser (slightly overlapping the mids) which some may find as a negative.

The casings where the drivers sit are different and the drivers are tuned differently.

The sound on the 555 is laid back and the 555 is not as airy and open as the hd558 or the 598. The 558 is also laid back and darker than the 598s.
The 558s and 598 are more refined and the frequency is smoother throughout relative to the 555s; the 598s are undoubtedly more refined than both the 555 AND the 558.

The 598s are brighter than the 558, the 598 are awesome for vocals, that is, the vocals are more up front and pleasant (especially awesome when listening to female vocals, I love my Adele :)and piano (awesome with piano), the 598s definitely shine with classical music as well. The 598 also excel in a wide genera of music including pop, jazz, classical, jrock, jpop, acoustics.
Bass is weightier on the 558s and tighter on the 598s (tighter bass is what I prefer, I like the tight punch). Not only is the 598 relatively bright when compared to the Senn signature, but it is funner to listen to as its mids are more forward than both the 555 and the 558.
The 598s have a Grado approach without the treble being overly sharp.

Like others have said, and I concur, the 598s def open up/come alive when coupled with an amp/ decent DAC. But at 50ohms, an amp is not necc to drive these babies.

A number of audiophiles have stated that: The sound from the hd598 are UNMATCHED by ANY other headphone. AGAIN, that doesn't mean they are better than, for instance, the hd800s but rather that they are unique, nonetheless, a notion is to be taken in a positive way.

Comfort is great on both the 558 and 598. BETTER than the 555/595. If you call Senn they will tell you that the clamping force is the same BUT I as well as a couple of others I know all agree that these are MORE comfortable, you pretty much forget that they are on your head (that says a lot especially because I wear glasses and I'm really sensitive to the pressure in that area).

If you are going to be playing electronic, house, metal then these headphones may be a bit slow for you, they could use some speed. If you want that, then I recommend the HD 25 1 ii. DO NOT get the hd25SP (easily mistaken since the picture is incorrect on amazon.com). The hd 25s are closed back, awesome for portable use and the no.1 DJ headphones in the world. They are also used by the broadcaster in NBA, NEWS etc (their version has a special mic attached of course).

If you were to do a AB testing bw all 3 like I did, you would definitely notice the difference, the HD598 is the clear winner in my estimation.

ALSO VERY IMPORTANT and thus should be noted, as stated by another reviewer there has been incidents in the past where there has been cracks that have been formed on the headbands of the 5xx series. Although Sennheiser essentially ignored the problem for several years they have INDEED taken corrective steps in manufacturing as of last year to prevent this. Further, they come w/ a two year warranty replacement and Ive heard pretty good things about the CSRs there.

Lastly, is it just me or do you guys also cringe when people/"reviewers" complain about sound leakage when reviewing an OPEN BACK HEADPHONE.

This headphone is an amazing product. I've used Sony XD400 and Bose Triports before. I'm not an audiophile. I just use it for listening to music and movies. It sound so great...so spatial....one word its heaven. It has controlled and well defined bass. Bass does not overwhelm mid/treble as in XD400. Sound leaks out but as advertised it is open headphone, so ofcourse it does. I can hear everything so clearly. Only thing is, the pins provided does not fit iphone properly unless you hold with your hand.