I've used my old HD600 for more than a decade with Cardas cable upgrade. First fed from a cheap CD player, then from a proper headphone amp (X-Can V3).
One year ago I've put some new (silver) drivers to HD600. Few months ago I've sold my HD600, bought a brand new HD650 and now using it with a stock cable.
Based on my experience, I can attest the following:
1) There is no such thing as veil on either HD600/HD650, neither old or new drivers had that thing.
2) Other than aesthetics and durability, cable upgrade does nothing to your headphones, period...
I respectfully disagree with you about the cable upgrade opinion. The Stefan Audio Art Equinox 650 LE is CLEARLY superior to the stock cable.
Had two pairs of Sennheiser HD 650's at one point. One had the stock cable, the other had the Stefan Audio Art Equinox.
My HeadAmp GS-1 headphone amp has two identical headphone out jacks. I plugged both pairs into the headphone jacks on the amp and shut my eyes and had my wife switch them out and not tell me which one had the stock cable and which had the upgrade cable.
I could pick the one with the upgrade cable out every single time with my eyes closed.
The one with the Equinox simply had a deeper soundstage, clearer mids and highs, and better separation of instruments.
Bass was very similar though.
The one with the stock cable sounded slightly darker, more congested, and the sound was flat compared to the one with the SAA Equinox cable.
We also swapped the cables around on the headphones...same results.
So it wasn't just my imagination.
I also found that the HD 650 also sounds different using the HD 580/600 stock cable and vise versa, but its a little more subtle.
The basic HD 650 sonic signature remains though, no matter which cable is used. Its just more refined with the SAA Equinox cable.