Koetsu black, lacking bass

dpoud, your tonearm is too light. You have to add head shell weights. To get it right you really have to get a test record like the HiFi News Test Record which will allow you to set the resonance frequency just right. I try to get it between 8 and 10 Hz. Koetsu's have fine bass but they are a very low compliance cartridge and require a heavier arm.

Mike 

Here you go  https://www.amazon.com/Tonearm-Headshell-Cartridge-Weights-turntables/dp/B01B8ONJVI

But I really suggest you get the test record!

Nice way to do it Uberwaltz. 
As a rule it is always better to by a tonearm on the lighter side as you can always add weight. It is much harder to take it away. It is also helpful if you can match the cartridge to the tone arm. A good reason to have two arms is to have a heavy one and a lighter one. Now with Koetsu's they only use two motors the one with regular Sumerian magnets and the one with platinum magnets. In the black and rosewood they use plain copper wiring. Everything else gets silver plated copper. The platinum magnet motors use to go only in the stone bodied cartridges. Then they have the rosewood signature platinum which I have. Not only is it less expensive than the stone bodied units but it weights 3.5 grams less. It is a much better match for my 4 point 14 which is a heavy arm with an effective mass of 19 gms. A stone bodied unit would push it over the edge dropping the resonance frequency too low. Using the 4 point's damping system might make it work ok. I have never tried it. I do not like to use damping and prefer to match the cartridge to the arm. I should play around with it (the damping) but I have yet to muster the inclination.