Hello !!
I also use 2 RB 500 and after I removed the silky mesh and the inner Frame with the wiremesh i can hear a little improfment in terms of Resolution and Dynamic,after i damped the whole lower Body with (non rottening) Dampingmatrial i hear a little less ringing at 3-6kHz ,after removing the Metalholeplate it was lacking high Frequencys, over 8 k so I put it back. In Terms of the Transformer it is good to know if they have the declaimed 600 ohm or if they are more in the 2 k Ohm Range.A Friend of me has a few older Rb500 from i think 2009 and they were at 2,3 k Ohm (Agilent Meter).After the change to the Lundahltranny,the Sound was worse, probably a Impedancemissmatch between the Ribbon ,Premierwinding,Secondarywindind and the Inputimpedance of the V72a which is even quiet high at 4k Ohm .We were not able to change the Ribbon yet, so we put everything back like it was .When these mics were plugged into a vintage,low Impedance Micpre,they sounded horrible,of course but when plugged into a modern Steinbergmicpre with a Inputimpedance of 2,5k Ohm the Sound was quiet good. Probably there happens a current instead of voltage coupling.Funny but if you keep the Cable short it works.When my 600 ohm Mics were plugged into the Steinbergmicpreamp they sounded worse, dull ,boring,and and zissy.With my V76 they sound great.(600 Ohm Mic into 600 Ohm Input,currentcoupling) So without a prof. Analysis of the Mic you have,i would not change the Tranny.Some Folks change the Lamination of the Tranny with better high Nickel one s ,but that changes the Impedance of the Tranny ,too,so thats not easy done.
I have a high respect for those who can crinp and attatch a Ribbon in the Motor of a Ribbonmic.I tried it a few Times,........... All the best,Lothar .
Greetings from Berlin and sorry for my bad written English