The way I understand it , is that SUT step-up ratio is inversely proportional to the input impedance, it presents to the cartridge
Your Koetsu has 5 Ohm impedance, so it likes "to see" the load, that is approximately 10 times higher, 50-100 Ohm
if the SUT (Cotter MKII L), is designed specifically for Koetsu, that means, it has highish step up ratio, likely 25-30.
BTW, I have Vosne Romanee and Koetsu Urushi cart myself.
I'm getting Ypsilon MC 10 L SUT (1:10 ratio) and 500 Ohm input impedance, but you can load it down to required by Koetsu 100 Ohm with additional resistors.
I'm going to try and see how it works