Normally I would more than tilt in favor of using an appropriate SUT into the MM inputs of your XP-25, with a MC cartridge. In the specific case of your My Sonic Gold, it's harder to make the case, though that doesn't mean an SUT won't sound different (and better), but the math doesn't lead to the usual conclusions about the differences. The oddity about the MSL Gold is that its internal impedance is a mere 1.4 ohms, but it's output is 0.5mv. Ortofon SPUs vary somewhat in specs but internal impedance of the SPU Wood is just 2.5 ohms with output voltage of a scant 0.18v. In that case, a 40:1 turns ratio SUT yields a cartridge load of ~29 ohms and an output voltage of 6.5mv into a 47kOhms MM load, pretty much perfect. You could even go to a 30:1 xformer and get a 52 ohms load with ~8.5mv output for the MM input of your pre.

But your cartridge combines very low internal impedance (just 1.4 ohms) with fairly strong output (0.5mv) for a low output MC cartridge. It's a lot more like the Ortofon SPU Synergy in that respect (2 ohms internal imp, 0.5mv output). In your case, if you applied a 40:1 ratio xformer for ~29 ohms cartridge loading into 47kOhms, output would climb to ~18mv! That's on the hot side. But your XP-25 is a very robust and flexible phono preamp. As Pass points out in their own manual, MC cartridge loading and gain is not a science, so any settings that work are fair game. So maybe you only need 53db gain with that kind of output. On the other hand, if you go to a 10:1 ratio xformer to get the cartridge load closer to the recommended 500 ohms, you would end up with a 470 ohms load with ~5mv output, spot-on for the 5mv - 10mv range you want to be for most MM inputs. Which approach will sound better, 10:1 ratio, 30:1 ratio or 40:1 ratio xformer into MM, or straight into the XP-25 inputs where you have resistor-based loading in increments beginning at 30 ohms? Hard to predict. But the cartridge sees a resistor and a xformer quite differently, so a 29 or 52 ohms xformer load may well sound much better than a ~500 ohms resistor load. Or maybe only a trifle better. Or not at all.

If you love the direct configuration, then leave it be and listen to music. If you really want to know, you just have to try it. There's no scientific answer. There is math to let you know the electrical context, but as Pass says themselves, MC cartridge loading is a compromise between the cartridge ideal and what satisfies the listener. As you approach a short circuit (low load number) the cartridge will at some point begin to sound muddy and then weaker on dynamics and average level until its unusable. Hence the old rule of thumb that the minimum MC cart load should be at least 10X its internal impedance. As you create more distance from a short circuit, tonal balance can/will shift to treble until you reach a point where no further difference is discernible. Some people like to run some MC cartridges straight into the MM 47kOhms if the preamp has enough MM gain. You never know *what* people will do!

I use outboard xformers on MC cartridges, or phono preamps with internal MC xformers as 1st gain stage, combined with variable resistance loading. I have not found a case where I prefer an all-active MC gain chain in recent years (circa 1974 the Levinson JC1-DC pre-preamp was a credible exception then), but then there are a lot of phono cartridges & preamps around now and I haven't heard *all* of them. Just more than a some.

Phil