Matt is correct in that the HD580 are extremely close to the HD600's in sound quality. They can be had for around $150 if you hunt some online (I paid this for slightly/lovingly used HD600's). I believe the HD590's are a variant and tonally differ from the 580's & 600's.
My other set of cans (which I often prefer) are AKG K501's (which I got brand new on eBay for $125). I find them to be more spacial and detailed than the HD600's. They also differ alot in the way they reproduce the bass end . . . they are not as dark and lack the 'boominess' of resonant bass response I find characteristic of the HD600. The AKG's still have excellent bass response, but it is much 'tighter' with more tonal characteristics.
I would recommend a good source as any defect will be revealed. Both of these will benefit greatly from a good headphone amp as well. Without an amp the AKG's will lack bass and the Senn's will lack detail, so you will not reap all the benefits that these superior cans offer.
You can always start out with the headphones and either build or buy an amp later . . . .
even I can build one 
You can read some reviews at Headwize Forum:
http://headwize.com/ubb/forums.phpBTW: listening tastes vary widely and are a matter of personal preference . . . what sounds excellent to one person may be terrible to another.
Nothing beats listening to them yourself.
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Topic: Good Headphones -- Advice needed (Read 3989 times)