Either you have a very special pair of older 650s or your hearing is entirely different than mine. I've owned both the black acoustic silk and silver acoustic {metal} mesh diaphragm versions of the HD600s at the same time and found the difference to be
very obvious.
I sold the black silk HD600s and kept the silver mesh version (both had brand new pads). I purchased the silver mesh HD650s after hearing them at a meet and as I couldn't then afford two relatively expensive (at the time) headphones, regrettable sold the like new HD600s with the silver mesh for $220
Out of curiosity I listened to the HD650 black silk version at a meet to determine if the same difference was apparent and it was. I have owned the silver mesh HD650s for years and would now not buy a pair of either with the black silk diaphragm...period.