Yes the HD600 are an exceptional pair of headphones, significantly better than the 590. What receiver do you own? Most of them will not do justice to the HD600s, although some of them have an specialized headphone amplifier (eg NAD) and are better (but still not as good as a seperate headphone amp).
On the other hand, the HD590 are of signifacantly lower impedance and much easier to drive, so an additional amplifer is not as important. Still, I wouldn't suggest getting these.
The HD580 is another option and is also very popular. I believe they use the same drivers as the HD600 and the main difference is in the case. The 580 is plastic, while the 600 uses carbon fiber and steel. Because of their similarities, they are quite similar sonically, but the difference is noticable, but some say that it does not influence music all that much. Being how I have never had the chance to compare them myself, I can't say whether or not this is true. I bought the HD 580s since they were $100 less.
More importantly though, is a headphone amplifier. I used a reciever with headphone output specs similar to yours and thought my 580s sound very good, until i got a seperate amp. The difference is really phenomenal. If you don't have the money to get the HD 600 and an amp, you could go for the 580 + amp. I think this would sound better the 600 without one. On the other hand, you could get the 600 now, and get an amp later.
Search the forum, there any many threads concerning this type of issue.