I'm new to Step Up Transformers

I checked and the Paua is indeed a moving iron cartridge like all the others in the Soundsmith line. 

It is moving iron, but it has a .4 Mv output.  

The approach of using a hybrid transistor and tube to supply the needed gain for low output cartridges is to reduce the level of noise from using tubes exclusively.   

The use of a SUT also reduces noise as an issue and adds the sonic signature of a transformer.  If you want tube only amplification (no transistor stage or no SUT) you have far few alternatives.  I mentioned Aesthetix earlier as an all-tube design that sounds good (if a bit noisy); another good sounding example is made by Lector (quiet for an all-tube design).

Yes, it's a moving iron but a very low output, similar to a low output Moving Coil. 

I have two low-output MI cartridges, one of 0.4mV output, which requires an MC input or external SUT, and the other with 1mV output sounds better that way (but could be connected via an MM input and the gain turned way up).

I have three ways of doing it - using the Musical Fidelity NuVista Vinyl and setting the input to MC, using the Quad 24p set to MC (with internal SUTs), or using an external SUT and the Quad set to MM. The last of these sounds the best of the three, so that is what I do.

Atmasphere is also all tube high gain phono, as I mentioned earlier. I’ve owned an MP1 for two decades upgrading along the way. I also owned an Aesthetic Rhea Signature but not the Io. The Rhea is amazing for its remote controlability but was a tad noisy compared to the MP1 when I cranked up the gain.