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.

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?

Dear jarret: """"  I also get some mistracking on high energy vocals.  """

The RSP is not exactly with the same Koetsu sounds signature that all previews models, as you noted has extended high frequency range but even that is different and I can't be sure if that " imbalance " in the mid range is a real imbalance or part of the cartridge it self or part of your own preferences on kind of sound.

What is an " imbalance " is that mistracking because the RSP is way better tracker that what you are experienced. Could be a non proper match with your tonearm, a faulty tonearm at the bearing or a RSP problem in its suspension and if this is the problem only Koetsu can fix that suspension and maybe they do it by free of charge.

Regards and enjoy the music,

I think a lot of us forget that there are other issues relating to cartridge wear, besides just the stylus wear. A common problem is the drying out of the suspension components that are internal to most cartridges. After sitting in storage for many years, this would be one of the most likely issues. It would/could also explain a lot of the issues that the OP is concerned about.
Sounds like a re-build would be a good idea.
I have personally owned a Rosewood Signature Platinum (RSP) along with several other Koetsu Cartridges.  I loved the sound of the RSP.  After considerable use, I needed to retip it and sent it to Soundsmith for repair.   I was extremely pleased with the results.  I have also send other cartridges to Soundsmith and each time was pleased.  Twice, I got one back with not charge, except for return shipping, when they said that nothing was wrong.  I suggest you go on line and send it in for evaluation.  They say there is a nominal charge for doing that, but you don't have to make a decision up front to rebuild it, you can get their opinion of the condition before making a decision.
Good points above, from people far more experienced than I am.
I would give it another 100 hours of real play, with not valuable records just in case, and only then send it out for evaluation. After ten years it might need time to wake up. 
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Koetsus are very sensitive for proper alignment.
Most think, they have done it correctly, but from my experience, most fail. And this piece of wood - which may be called tonearm from some - is hardly something done right.
I agree with Syntax. Koetsu's are extremely sensitive to set-up. My suspicion would be azimuth.

I suspect that very few cartridges, even Koetsu's, have MC motors fixed into the bodies at a precision exceeding 1 degree of arc. After many hours of play and fiddling, I made my latest correction: about 8 minutes of arc, and the Koetsu just SNAPPED into focus. It's a bit of a trial to get there, but worth it. For completeness, I use VTA slightly tail down and 2.1 g of tracking pressure.

Also, consider those loading resistors. Each brand and each type of resistor has its own sonic signature. To the amazement of an IEEE bible thumper, I demonstrated sounds ranging from sublime to unlistenable just by changing one (expensive) gain resistor in the preamp (the most expensive was worst).  I like Nude Vishays in my system, but you might also consider Mills wire-wound resistors if sibilance remains troublesome.

About "demagnetizing" your KRSP, I would definitely avoid any active device to avoid overdriving the motor. I use the "demagnetizing" tracks on the Analogue Productions test record, specifically, bands 2 and 3 of side 2.

Good luck, and hope that helps.

Thanks to all for your responses. I just got a Denon 103-r as a backup which I will install this weekend for comparisons.

The Reed on paper should be a good match for the Koetsu, at least in terms of effective mass at 22g. I also tried adding a 3g headshell weight but thought it sounded better without it.

I agree with the loading resistor comments as well. The TX2575 I am currently using is so transparent and quiet it is scary but may be too incisive and edgy for my ears. I will go back to the Shinkoh after the Denon trial and will continue to tweak the azimuth. I found the 45rpm Nat King Cole Mono album excellent for dialing in azimuth.

Tracking lighter than 1.95g just increases the thinness and sibilance.

About 6 month ago I sent my koetsu silver onyx to Koetsu Japan, as it developed a bent cantilever [ it actually still sounded great] it cost me half the price of a new one,$4,500 aus. for a complete rebuild. sounds superb, not even run in yet