Having a different internal sound signature of your head shouldn't change much. It's like asking someone how they see "red". Instead, just like a color blind person will see things differently, only someone with hearing deficiencies would hear things differently. The thing is, I think most people have slightly different ranges where they
Honestly I think that the main difference is because people have different hearing ranges. In other words, if HD600s have a perfect FR up to say 15khz, but a horrible recession to the extreme high treble above 15khz, then someone who can only hear up to 15khz very sensitively will find the HD600 perfect. If on the other hand the SRH940 has a somewhat colored FR up to 15khz but great above 15khz, then someone who can hear up to 20khz sensitively will find the SRH940 sounding more natural.
I do not find the SRH940 to be perfectly neutral either. I found my DT880s much much more neutral -- yet instruments sounded blurry, muffled, yet sibilant, and like they were dead and completely dry. Not even slightly realistic. Not because the sound signature was wrong, but rather it wasn't detailed enough, and it didn't present the upper frequencies correctly. The SRH940 gets 15khz+ right to my ears, and that's not something I've found in any other headphone YET (not DT880s, not AD2000s, not HD650s, etc.). I'm still looking.
HD650 I prefer for piano, but I find it way too muffled and unlifelike even with equalization to enjoy violin and brass the same way as the SRH940 presents it. SRH940 sounds astonishingly close to real life where the HD600/650 cannot - far upper treble 15khz+.