Well, I
own both cans and the answer is an absolute 'YES', the HD650 really is worth 2.5x the HD598. But you won't get to that place with a quick listen at a meet or in a shop; you need to own the 650s for a period of time and acclimate to their sound.
That said, I would weight the opinion of someone that has listened to a LOT of different headphones, as opposed to worrying about ownership or test driving. When you own one or two pairs, you really don't know what you don't know until you hear other products (say, like, at a meet or with friends).Ok....I own the sennheiser hd650, sennheiser hd595, and sennheiser hd280 pro.....I used to own the hifiman he-400, but gave those to my girlfriend....I honestly love my hd650's! Like I said, I owned 4 pairs, but now own 3....I've heard many different models already, including Stax SR-009's....I love my hd650, and yes, they're worth the extra money over the hd595...and yes, I've heard the hd800 a few times already, and have even listened to the audeze LCD-X, and yes, the hd800 sound anemic, more natural? Alittle...better detail? Not really....I preferred my hd650's the most! Over the audeze and the hd800....The only thing that made me almost faint when I listened? Was the Stax SR-009! That's the truth.....people that blow $1500? You must really be looking to be popular, because that ain't worth $1500....The hd650's are incredible! Trust me! Try listening to the hd650's with a binaural recorded CD! I have over an hour and 15 minutes of pure binaural recordings, and I'm ready to pass out EVERY TIME I listen to this CD with my hd650's! Good recording with the hd650's? Sounds good....epic recording? EPIC SOUND outta your hd650!