Thanks again for your insight!
One more thing, that person who is selling me the HD650s also has the HD598s for sale for 125€ (bought from Amazon last October). I arranged an audition of the two headphones with him, and he says that if both end up pleasing my ears and I get both from him, he will make me a "nice discount", his words. By the way, both headphones come with aftermarket cables, don't know if they're better than the stock or not though. The one that comes with the HD650s is a 1.2m Furukawa.
My question is: would I get any benefit from buying the HD598s too? I know they are technically inferior, but can they compensate for the laid back treble in the HD650s? Would their airier sound and larger soundstage (?) give me an advantage in gaming over the HD650s?
Then there's the low impedance advantage. I could almost plug in the HD598s to anything, and they'd still sound pretty good, I think.
The HD600s also enter the equation, as I can get them from Amazon.fr new for about 270/280€.