Dear neonknight, I do not follow your logic, at all, in arriving at the conclusion that you need a SUT. Your internal argument seems to be based on wanting to provide less of a load resistance for your Koetsu, something higher in value than 500 ohms, based on what you wrote. But adding a SUT will REDUCE the load "seen" by the cartridge, by a factor equal to the square of the turns ratio of the SUT. For example, if you run a 1:20 SUT into the 47K ohm fixed load resistance of your NuVista, the net load seen by the cartridge will be 47,000/400 = 117.5 ohms. This is apparently lower (or more of a load) than you desire. (See also below about why not to use a SUT into an MC phono section capable of 70db gain.)
You state that the Urushi sounds best to you, or most open, with the 47K ohm load afforded by your NuVista at its MC inputs. Why not go with that? There is absolutely nothing wrong with that. And finally, if you are going to use a SUT with either of your two phono stages, they must have an alternate pair of inputs for MM cartridges (lower gain) that provides at least a 47K load resistance if not higher in value. With 70db of gain from your NuVista MC section, using a SUT would overload your phono stage and probably would sound bad, besides also reducing the net value of the load on the cartridge, thereby loading down your cartridge (see above).
As an aside, I have lately been using 47K as a load for all of my MC cartridges; it sounds excellent. I historically used 100 ohms as a load for my Urushi, and I was also satisfied with that for many years. The value of the load resistance should have at most a very subtle effect on apparent frequency response of an LOMC cartridge, most likely to be heard at the very top end of the spectrum. It's more of a "feeling" than a measurable roll-off.