When you use a SUT you should use short, low capacitance cables. Under 1m for sure, ideally 0.5m or as short as you can go. It has to be low capacitance - for example, the Audioquest LP / phono cables, and their Bridges / Falls IC cables are good, but their higher end Elements interconnects are BAD for this. Cardas interconnects typically have extremely low capacitance and are good for this.
I’ve done the 10x SUT thing for a Benz LPS with 0.34mV and 470 ohms MINIMUM recommended load (it seems like it wants a higher number), not too unlike your SS but MC based. Tried a few SUTs, 8x - 10x. It was an OK result, not stellar. I feel like this kind of cart is a tough match to a SUT (all my other carts love a SUT). With the LPS, I got a slightly better result from a Hagerman Piccolo2 which is a JFET headamp that has amazing gain & loading options and only costs $289. But the best result I’ve gotten from the LPS so far is on a Hagerman Trumpet MC, which only costs $1100 new and has an improved balanced Piccolo built in. The MM stage is tube based and (mostly) balanced circuitry. It’s not as quiet and refined as my VAC phono stage (which is SUT based), and the chassis & wall wart are clearly built to its low price point, but for that tough-match LPS it gave a better result than the ~$16K of phono stage & SUT.
The resistor mod to you Luxman sounds like it might be a great idea though.