I'm new to Step Up Transformers

As a person who 50 years ago did build a speaker without knowing anything about crossover design (not speaker design because I blindly followed information contained in a magazine article), I’d say Atmasphere has a point. But if you read the pdf available at Jensen, you see they recommend a rather complex network of R and C to get correct loading. Almost no one actually does that in implementing any of the dozens of different SUTs mentioned on this and the many other threads devoted to SUTs. So what do all those reports mean or how should they be interpreted? With more than a grain of salt, I think.

But if you read the pdf available at Jensen, you see they recommend a rather complex network of R and C to get correct loading. Almost no one actually does that

You are talking about a Nobel network to dampen any ringing in the transformer.

The Zesto Andros has the Nobel network included in their circuit, along with loading options designed specifically for that transformer/zobel. I know this because I installed an in field factory upgrade on a Zesto Andros a few years ago.

The slight complication here is that the Paua is a moving iron not a moving coil so the behaviour will not be at the same as LOMC's.

Soundsmith can advise on which transformers are appropriate.

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.