If it turns out you prefer the HD600 sound more, you can just buy the drivers and replace the HD650 ones. You get to keep the 650 look with 600 sound.

Wouldn't you have issues with driver matching if you do this?  Having contacted Sennheiser regarding this issue, they sell the driver enclosure/capsules individually and don't do any driver matching.  Then again, this is Sennheiser we're talking about, who seem to have far better consistency than just about any other headphone manufacturer in the business. 

I wonder how much variance one can expect from individual drivers...its the only thing stopping me from buying two capsules straight from Sennheiser to "drop" into my HD600. 

So to me the whole 'veil' thing can be attributed to FR related things.
THD and CSD is similar for HD650 old and new pads ...

Totally disagree. Correlation is not necessarily causation. The HD800 and other phones with massive troughs of recession from 1khz-6khz don't sound veiled. Plenty of phones that have the same amount of upper mid and lower treble energy as a worn out 650 don't sound veiled. You're just equating increase in treble with more clarity. Not the same thing. The Senn 'veil' is like having a good recording with too much noise reduction applied versus no noise reduction. It's what I call 'greyness'. You can apply EQ to most veiled phones and drivers like these and the veil will still be there.

Talking about this veil is getting to be like explaining plankton to someone who hasn't heard it from an O2.

Better to compare the HD595 to 598. Pads don't mean squat here and you can physically take apart the drivers and inspect them for obvious photographic proof of differences as to why one is veiled and why one is not.

Veiled:
http://www.innerfidelity.com/images/SennheiserHD595.pdf

Not Veiled:
http://www.innerfidelity.com/images/SennheiserHD598.pdf

Oh I hear you. Or maybe not, as my HD800 listening might have deafened me by now.  You can certainly 'veil' a headphone via severe FR manipulation. That may be a 'contributor' here and some other cases, but I think there is probably more to it going on with the Senns. My experience with the lower end line definitely and absolutely supports that.

On those, the main difference lay in the thickness of the paper damping over the drivers.

Pretty sure the box change wasn't the actual revision. Sounds like there was one in 2007 and maybe (I'm guessing not) one more, but we'd have to verify.

Well, relative to the Stax stuff, anything clams like a medieval torture device.

Relative to Beyer (DT990), HD6x0 clamp lots less. Relative to the AKG, I would say it's somewhat sameish. If the HD800 clamp, Anaxes HD800 lost their virginity.