[Monsterzero]

what type of sound are you preferring?
Bass heavy?
Rockers?
Neutral?

Bass=Philips Fidelio X2 no amp needed
Rocker=Grados depends on model,but generally same as above
Neutral=Senn 600s amp needed,not necessary,but it REALLY helps

 

 

[Durza]

So, my understanding is that the hd 600 will be a pretty different experience from the hd 598. The hd 600 has some differences from the 598, so it won't just sound like a improved 598, but it will be a different experience(Disclaimer: I don't own the hd 600's and have never even heard them). Now, I own the 598's and they were my first decent headphone, from there, i have moved over to the akg k7xx, and I am happy with the upgrade. The k7xx will be better in every way(except comfort and soundstage) vs the 598. The k7xx isin a grey area where you can get away without an amp, but amping will benefit them. Also, the k7xx is only 200 bucks, much less than a hd 600 My newest headphone is the fostex t20rp mk3, but it is yet to come in... The fostex lineup may be worth checking out because they are planar and supposedly have fantastic mids if that's what you are into. My suggestion is to do some serious research on:
Akg k7xx
Sennheiser hd600
Fostex T50rp/T40rp/T20rp MK3
If you really want the hd 600's there is a pretty good amazon deal here: 400 bucks, but you get a 150 dollar amazon gift card with them.
http://www.amazon.com/gp/product/B0199APD94/?tag=dealnewscom&m=ATVPDKIKX0DER

 

[csmx123]

I recommend you to stay with 598s and be happy with them 

 
They are one of the most comfortable phones I've ever had, right after the lambdas.

 

[desik]

The descriptions on this guy' page are quite enlightening: http://www.head-fi.org/u/151552/alexroma
 
I'm frustrated I didn't found him earlier.

 

[fquails]

I found this article very helpful to simplify my search for headphones:
 
http://www.head-fi.org/a/headphone-buying-guide
 
In addition, I auditioned many headphones, including Grado SE series (too bright for me), Senn HD 700 (too bright for me), Senn  HD 558 (sounded hollow to me), Hifiman HE400s (Fantastic sound, but massive size and heavy weight uncomfortable for long listening sessions), and the Senn HD 650.  I loved everything about the HD 650 -- sound signature, comfort and looks.  So now I have 2 of them, one for home and another for work.  Definitely need a decent amp, so I got the Schiit Magni 2 amp (one for work and one for home).  For portable use, I have the Fiio e12a.  I don't listen to loud music--mostly within 65dB - 75dB, so my amps are more than sufficient for me.  I listen to everything except rap, hip-hop.   My preferred music are jazz, classical, folk, acoustic.
 
Best way to really pick a headphone is audition them for a period of time with your preferred music and existing equipment.
 
Good luck!