I have pretty new 650s, drivers look just like yours. Bought them directly from Senn. Non modified in any way except for stretched headband. Accidentally just listened to it this morning, after maybe 2-3 weeks break, out of properly warmed up Yggy/Rag combo. Good but nothing special and measure quite a bit worse then how yours appear to look.
   Circular cutout in front and absence of foam in the back sounds simple enough but why amazing engineers at Sennheiser couldn't figure it out for so many years of constant improvements, I mean seriously?  You know I trust your expertise but are you sure this pair didn't come in a golden box directly from their CEO? I mean Amazon must have some connections and maybe some secret agent out there is tracking all your purchases related to audio?
   All kidding aside, should I send you my pair for comparisons? Who knows maybe I have a lemon here and always misjudged all the 650s because of that? For about a week I had 1st generation 600s right before this pair and not much difference neither in measurements nor in listening impressions, there was no direct comparison though. Oh man, and how am I going to sleep tonight
     

The penguin approves of this desription:

The Amazing Super Duper Underrated HD650. A Headphone That Kicks Serious Ass.

An AKG-fan agrees with you.

At low listening volume, the HD650 is crap. At high volumes and tubes it's basically god.
But what isn't at high volumes and with tubes. 

650 si more refined IMO and I've the same problem than marv' with the vocal shout of he HD600.

That's what made me buy the HD650 over the HD600 back in those days (aug 2004)

The other site can shill all it wants, people's ears don't lie in long term.
HD800 and HD6X0 have become seemingly quite popular.

From car analogy HD650 is like Saab 9-5 and HE400 is a Kia 
It is tempting for a novice to buy a new Kia vs a used european car in these parts of the world.