If you like the sound of the hd600. There are lots of alternative you might want to try out first before you get the hd600. HD600 is characterize as an open back neutral sounding headphones.
You can give the AKG K702, the audio technica AD700x, AD900X, or the AD1000x even the ad2000x a try. These are good alternatives. The audio technica are more open and more airy sounding and have the most wide soundstage then the hd600. The AKG is very clear and neutral sounding and is a good alternative to the hd600.
Can also give the massdrop HD6xx a try as this are the same as the HD650. More warmer sounding then the hd600 which alot of people like compare to the hd600.
Also if you gonna buy especially the sennheisser headphones try and splash some extra cash on a good dac amp. You can unleash more potential from the headphones.
If you prefer more bass, more warmer sounding but still able to maintain that wide soundstage, can also give the ath-a990z, ath a1000z a2000z a try. These are closeback version of the audio technica AD series which is the open back version.
Also, give the beyerdynamic headphones a try. Their dt770, dt880, dt990 is a legend and is highly love by lots of audiophiles. Do give them a listen and then decide if you still prefer the hd600.
There are lots of good alternatives to the hd600 do give them a try. The sennheiser headphones are harder to drive and really do need a good amp/dac to make them sound really good. Although they already do sound very good even without good dac/amp.
HD58X is not the HD600.
HD6XX is the HD650.