In your private email to me you described the capsule of the mic which you compare the C12's capsule to as being in a C414EB. It can easily be mount it the C12, and you then can listen for sound differences, before you start screwing around with the mic's electronics.
For all you know, you may have a flat-dish CK12 in the C12, which has markedly mellower top end.
If the CK12 from the C414EB still sounds odd in the C12, then I would attend to the coupling capacitor and other electronic parts.
Also make sure that you run the C12 at roughly 120VDC B+ and 6.3VDC heater.