Koetsu RSP Sound Problem

I recently bought a Koetsu RSP from a very reputable dealer. The cartridge is about 10 years old (based on serial number) and sat unused, after about 150 hours of initial use, for almost a decade.I now have about 100 hours on it myself.
It is mounted in a Reed 2A 12" Cocobolo arm on an SP-10 and runs through a Coincident phono stage (retubed with NOS Mullards) and Coincident Frankenstein monoblocks with Takatsuki 300b’s.

The RSP sounds very good with excellent bass and surprisingly extended treble but has leanness in the lower midrange and somewhat accentuated upper midrange (sibilant). I am fairly confident of the alignment and have tried VTA’s from slightly above to slightly below level, adjusted azimuth visually, electronically and sonically and have run VTF from 1.92g to 2.00g. I have also been adjusting loading and have tried 100 ohms with both TX2575 resistors and Shinkohs. The Shinkoh provides a slightly fuller midrange but the 2575 has a transparency that is hard to ignore.

No matter what I try and I can’t remove the midrange imbalance which is very contrary to the Koetsu RSP reputation.I also get some mistracking on high energy vocals. I am considering sending it for a rebuild but thought I would post here before shelling out the $$$ to see if anyone else has experienced this issue.