Let's get back to the original question and a possible answer:
This 1999 u87Ai was serviced by a reputable music gear dealer that included tightening up the capsule. Would it be possible such a service could have made the pre 2000 u87Ai now bass-starved and too midrangy?
After reading your PM with the relevant quote from the service technician ("tightened up capsule & capsule mount") it seems to me that, if anything, he did
not manipulate the diaphragm tension. He may have tightened the four backplate assembly screws which would compress the spacer in between, which would alter the capsule's acoustic network, making it mid heavy. But that is mere speculation, as the description from the technician is vague.
If not happy with the sound, I would just abort the purchase rather than spending more time and money to have a mic you love.
P.S.: I tightened the thread to bring the meandering subject back in focus. I apologize if several posts had to be deleted in the process.