The UM92 was build upon request of the former Gefell US distributor G-Prime. However it was nothing else than a slightly modified UM57, only difference was the output x-former that was taken from the MV amplifiers. Aside the low grade metal composition, the core was/is not able to get any resolution an M7 requires.
The UM92 was original shipped with an RFT Neuhaus EC92, even selected that tube is low grade compared to a Telefunken or Philips/Mullard EC92, I prefer the Tefefunken but the Mullard is equal in noise specs.
A simple tube swap might help, but without proper changes/modification the full potential of an Telefunken tube nor the M7 won't be usable.
After Gefell could not get any longer the RFT EC92 the EF86 was chosen as a replacement, also the regulator inside the mic was replaced with an updated PSU as well as some crowbar EMI/RFI filters. There are many problem that come with the EF86 in a relative low voltage set up, but the biggest of them is that Gefell choose a Svetlana EF86 tube.
Both mics have the x-former connected to the cathode resistor acting as a feedback to stabilize the load.
In order to move the UM57/92/92.1 closer to the full potential of an M7, here are some steps that will help.
Replacing the stock 50K plate resistor to 100k with a 1uF (UM92.1 1.5uF) high grade capacitor.
Replacing the stock x-former with a high grade one and grounding the x-former instead of laying it on cathode potential.
Also adding a 50uF cathode cap, and a 1G high quality grid leak as well as polarization resistor.
A good NOS tube of western origin!
Changing the C12 style remote to an M49 style and most of all replacing the crummy PSU of the UM92.
Hope that answers most of the questions,
Best regards,