I'm new to Step Up Transformers

I am constantly surprised when others report they hear important differences between two different load resistances, using LOMC or LOMI cartridges, that are numerically very close to each other on a scale of 100 to 47K ohms. Like 725 vs 525 ohms or 800 vs 1000 ohms.

You make too many assumptions.

I agree on the MC that if small changes are audible, as Atmasphere has pointed out, it is more likely that the target phono is reacting.

However it is a different electrical model with a MI such as the Soundsmith.

I ran the Paua through 4 phonos in one afternoon, including the Soundsmith phono, and 3 others, both transformer based inputs and solid state and small changes in loading 700-800-900-1000 ohms were quite audible in all 4 phonos. If I recall correctly all 4 phonos sounded best with the same loading, in my case 800 ohms.

Paua owners please be aware that the Paua has undergone electrical changes through its lifetime, the inductance and other design parameters have changed over time so it pays to experiment - or follow the guidelines by Soundsmith.

In my experience with 4 different phono stages. I have always preferred the sound of using an SUT into the MM stage vs the built-in MC stage. Whether the MC stage has built-in step-up transformers or uses SS for gain, the result is always the same for me, SUT into MM always sounds better to my ears. The only caveat is that you must invest in decent phono interconnects for the SUT. 

'I have always preferred the sound of using an SUT into the MM stage vs the built-in MC stage.'

This has generally been my experience too.

My Zestos offers only 2 load settings that are in the range to suit the Paua (minimum recommended setting of 470 ohms):  400 or 1000 ohms.  I have set it at 1000 at the moment but it requires more listening. 

Hello, I don’t think many people understand your question and what your friend is telling you. I personally own 3 preamps that have excellent phonostages. I own 2 high level, very expensive external phonostages that have MM and MC settings. Both external phonostages have Lundahl transformers to step up the signal for MC cartridges.  The point your friend is making, is to try various other Step up Transformers via the MM section of your phonostage. I personally own 7 or 8 SUT’s and have discovered they all have their own unique sound and it takes time to match the perfect SUT with your cartridge. The more options you have, the better the results will be. If you are happy with the built in MC section of your preamp, then enjoy it but if you want to hear things change in sound, just like changing the cartridge affects the sound dramatically, trying different SUT’s can make the sound better or worse as well. I personally am a fan of options.