sounds like someone who's never heard hd800 S with a proper amp (or possibly at all)
The hd800 s has I believe 1/10th of the harmonic distortion as the hd600's, with an even better frequency response. It's made with higher end materials, and is incredibly comfortable.
There is definitely a large sound difference in the two headphones, some prefer the laid back sound of the hd650's, while some prefer the extremely analytical sound of the hd800 S. In fact it's so analytical it makes many recordings that sound great on the hd650's sound like utter garbage.
Yes, some are just audio snobs, but some people actually need headphones that are that analytical, and yes they do sound much much much better. Not 5% better, much more. Looking at specs dont tell you the whole story with the control that the drivers in them have.
For example listening to a piece with for say 30 parts going on at once (like a mic for each drum, multiple guitars, microphone for each of a bunch of backup singers, full works) the bass on mid fi headphones will be correct. The right note, but when you listen and focus on that note on the HD 800 S, that note is more accurate, more controlled.
listen to a $300 sub speaker, and then a $1000 sub. Pretending they both have the same range in Hz, the cheap sub will hit all the right notes, but the more expensive sub will be more precise, less muddy, and quicker to respond. Same goes to headphones.
Its also a personal thing, some like to listen to music, some people like me who record music and mix it need it to be precise and not hide any minute details.
If you can't hear a large difference between hd650 and HD800 S, either you shouldn't be spending a lot of money on headphones (bad ears) or they weren't driven properly (poor amp choice, bad source, cheap DAC, things like that)
There is a very large difference in sound, whether they're for you or not is not up to me, but you can't claim that they barely sound any better.