Thanks jae for a rational reply
I agree with you : my EQed HD600 are possibly my favorites and the closest thing I have to the supposed "truth / naturalness". I also love my Nighthawks : definitely not neutral-sounding (even when EQed, because of internal resonances I guess), but surprisingly good (even better than the HD600 on some types of music where "truth" is not an absolute requirement), with decent soundstage, and a bass and imaging to die for. So the two of them are quite complementary.
Like already said, I'll definitely keep an eye on DCA stuff (hoping that the Euro rises again in the near future, lol). I'll try to listen to them in my city, but here in France it's not going to be easy. I'll post in french audio forums and I will try to set up a listening session at my place (I have several headphone amps and my own software for dual listening sessions). Who knows? Maybe I'll get lucky.