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.




jarrett
Before you do that, because its been sitting around for awhile, I'd try demagnetizing it with the use of a fluxbuster...just be careful that it is recommended.  Some cartridges (don't know about the Koetsu) has internal wiring that could melt with the use of the f/buster, because of such thin wiring.  I think its worth a shot with that caveat.  My Benz LPS really oppened up with its use.
(1) ~2g seems perhaps a bit high for VTF, at least it is for my Urushi.  Have you tried 1.5 to 1.6g? (For all I know without looking it up, the VTF you are using may be the recommended one.) In any case, I would put a few more hours on the cartridge before doing anything drastic, if you think the history is accurate.  On that score, have you examined the stylus under magnification to verify its "like new" condition?  Test LPs with "break-in" cuts on them can be very helpful to wake up cartridges that have been long in storage.  The Cardas test LP is one that does have that feature; I use the 2a, 2b, 2c sections for that purpose.
(2) Have you got a fix on the inherent sound of the rest of the system, using some other cartridge?  Can you be confident that the tonal balance you ascribe to the Koetsu is in fact due to the Koetsu?