So when is a SUT really needed?

Simple response

Your Moon phonostage has a 66 max gain setting.

You don't need a step up for any typical cart used for the table you have.

The 60/66 setting will cover just about all the popular choices for your table. Generally, they're not going to be lower than .4/5mv. Maybe .3mv would be the low end? Spec's and real world results allow for -.1/2mv variance.

The legendary Denon 103 is .25mv. A little low for your setup. 70db or so would be optimal. You could use without step up, but performance won't be optimal.

In general, you need a SUT in case of low output voltage MC cartridge if phono  pre-amp is not designed to work properly with that lower voltage, ie designed for higher output voltage MM cartridges only.

For example, I’ve used one (Electrocompaniet) with a Denon dl103 low out MC cartridge to several MM only phono sections over the years.  Not much volume/SPL possible otherwise.   

@jay73 

This is a case where the cart really should come before the horse.  Whether or not you need a Step Up Transformer (SUP) critically depends on the output level from your cartridge, and what input levels your phono stage can deal with.

By convention, cartridge output level is measured in milliVolts when playing a 1-kHz sinewave with an amplitude equivalent to 5-cm per second.  Many test records have a track like this.

Moving Magnet (MM) cartridges typically have an output around 5-mV, which is suitable for almost anything having a phono input.

Moving Coil (MC) cartridges usually have much lower outputs, from 1-mV down to 0.2-mV.  At the low output levels, the cartridges are known as Low Output Moving Coil (LOMC) and they need much more help to increase the gain.  All other things being equal, increasing gain also increases noise.

Gain options include phono stages designed for MC, SUPs, or both.  A SUP is just a transformer with two coils of wire, that converts voltage as the ratio of the number of wire coils on one side to the number on the other.  In a perfect SUP, there would be no power loss - for any increase in voltage there is a corresponding decrease in current.

You can replace a SUP with a powered amplifier to boost the output voltage - a Head Amplifier.

In general, there is a trade-off between output and trackability - the ability to precisely follow the groove.  For good trackability, you want low effective tip mass, which means smaller parts and less output.

There's another subtle consideration.  MM and MC cartridges must generate electricity, which reduces trackability as generating power electrically 'stiffens' the stylus.

More recently, practical optical cartridges have become available with a massive 70-mV output.  They are separately powered, have very low tip mass, very low noise and very good trackability.  The downside is that they respond to position, not velocity, so special equalizers are needed to replace the normal phono stage.

Once you have selected a cartridge and your phono / equalizer equipment, you will be in a position to judge whether a SUP makes sense for you.

Hope this helps

@jay73 

Simple truth is just about any solid-state MC preamp will have the facilities to match to just about any MC you want to use with it. There are certain exceptions where MCs have a very low internal impedance and as a result very low output voltage which can be accommodated by matching it with an appropriate SUT or a transimpedance type preamp. Otherwise choosing an SUT over solid-state for most MCs is purely subjective or a matter of taste.

"Need" with a MC Phonostage: No.

Correct, not power cable, just line in and line out. It is passive. So what's the point?

I was wondering that myself, added a Zesto SUT to a Rega Naia rig, and an EAT eglow petite. It changed the sound to something fuller, more organic. I was surprised, as I did not expect anything, given that it is just a passive transformer. I am in the more skeptical league of fine audio adjustments, never heard a difference between any cable. The SUT was about as much a difference as between two carts, or suboptimal and optimal impedance/capacitance setting on a phono state for MC/MM cart, or maybe +/- 0.2 g VTF adjustment on a cart.

The sound was not objectively bad/worse without SUT, it has a different flavor. You may like it better with or without. I don't think there is a way to predict the outcome.

my 2c, or rather 300000c. LOL!