black koetsu sounds weird

I have an old Black Koestu on a Linn ITOK arm on an Oracle Delphi table. For phono stage I use the rega fono. I push this through a Meitner pre amp and amp. I condition power with PS audio quintette and a dedicated circuit. Some days its OK and some days just downright miserable and thin. Any thoughts anyone?
Koetsus don't sound thin....ever. So your choices are that the suspension has hardened, and it is time for a rebuild, or that your setup is not correct. Check VTA, lowering the back of the arm may help. Double check offset angle and tracking force as well and perhaps reduce the anti-skate a tad. You may also want to double check the setup on the Oracle while you are at it.
If some days it is sounding ok , it may have magnetic particle buildup fouling the movement of the cantilever . A friend with a steady hand can sometimes remove this with a piece of bluetac on a matchstick under a microscope .