I'd say those were mint and the pads look to be in perfect condition too. The cable is indeed the only known place where they do fail and it is something which Stax took care of with the Mk2. Using them with an extension cable will increase it's life by giving it ample slack.
I always hang mine for the headband on a Stax stand and after 7 years (fuck that's a long time ) the elastic band is just like new.
It's more the amps that they haven't been so good at repairing but even Stax Japan makes some blunders like my SR-Omega/007 hybrid. Easy to fix though...
They won't fit in a Lambda chassis but I'm sure Smeggy could make something for you if you asked nicely. Without retracing the rather odd E-10 circuit it is hard to know why the caps are there. Probably some compensation but with Koss, you can never be sure.
The change is indeed very minor but there is an audible difference for some strange reason. For instance I've run the SR-007@800V with little to no change.
I never studied the ESP10 circuit in detail so I can't be sure what they did nor remember the values. The original plan was to ditch the cups and build something new and just use them with Stax Pro bias but then I got an offer for them and...
There plenty of them made but they aren't everywhere like L-Pro's.
In other news, I bought a SR-404 today so I feel a Sigma modding session happening soon...
I just removed the caps from inside the housings and converted them to Stax. It was a time of great abundance of headphones so I didn't spend any real time with them...