Hello. May I have a few cents??
The first thing I would like to do is congratulate you on the good research up to this point. I believe that you have stumbled on some of the best cans in the hobby. That being, said, I think alot of this decision is based around the rig you are putting together. Overall, I think I'm going to be simple and say the 595. Please let me explain...
The HD580 and 600 are the original Sennheiser audiophile and studio headphones. the 580 was designed in the mid 90s, while the 600 in the late 60s. When released both cans became the standard by which all other good cans were judged, as a result they are well known. Some of the best headphone rigs out there have been built around these two. For the most part, both cans can be described as hard to drive (meaning they usually need an additional amplifier for decent dynamics and speed) neutral. Both cans are generally thought of as un-colored, laid back, and a bit veiled in their presentation... imagine sitting 15 rows back.
The 595 was built to be a response to the criticisms against the audiophile line up. It is much more up front, easier to drive (with ipods and sound cards), and has very good speed. The cans can be described as very good all around, with all forms of music. My personal gripe with them is that they sounded far better from my soundcard than the HD650... but once I added a n expensive amplifier and DA converter to my system, the HD650 easily sounded better. Additionally, they can be a little dry compared to the full/rich sound of the HD650.
The HD650 is Sennheiser's best current can. It is somewhat difficult to drive. In making the can, Sennheiser wanted to built a can that the listener would fall in love with, and this they did well. It has an exaggerated bass and very soft treble, but it comes across as authoritative and very refined. The can is less veiled than it's predecessors, but still very difficult to drive nonetheless. I wouldn't really recommend this can unless you are willing to pay the money to buy the components that would make it worth while. Like I said, out of my sound card, i liked the HD595 better.
The HD555 is not really an "audiophile can" as much as it is a "mid-fi" can. It doesn't have the dynamics, speed, sound stage or preciseness of the 595. but it sounds very close. It's sound signature can be described as the "little brother" of the 555. There isn't anything too remarkable after that... If you have 100 dollars, the 555 is a great bargain, but if you have 200, I would certainly recommend the 595.
The other thing you might want to consider is the 325i. It is a hard hitting, super fast, bright, but impacting can that can get one's foot tapping quite quickly. It is not really as "audiophile" refined, or neutral as the HD600/650, but unless you are really picky about the texture of drums, I think you'll enjoy the 325i more for rock.
By the way, I currently see the 595 for alot less than the 230 you are about to pay on various places. just do an Amazon.com search.
http://www.amazon.com/gp/offer-listi...216285-5228471