So when is a SUT really needed?

I have Two readily available Phon' options to not use a SUT or a Head Amp, and my Third Option Phon' to not use the SUT or HA is ready for myself to pick up.

I also have a MM Phon' that requires a SUT or HA for my MC Cartridges.

I am always Wed to the SUT or HA as the method to use the available Phon's and am keen to learn how the New Phon' will be an influence in the home system. I have heard multiple hours of a version of this Phon' through the dedicated MC Stage and can assure I have been seriously impressed. My own system is the next challenge to have the experience.

SUT's and HA's are for me, able to inject a variety of Tonal Qualities that I am attracted to as the End Sound. For this reason I like more than one influence and mix it up across different SUT > Phon's  + Different Phon's with a HA, where thye remain in use as a certain configuration for quite a few hours before a change is made.

Only occasionally does the dedicated MC Stage get used for an extended period, usually a few Tracks is enough to want to return to the SUT > MM Stage.

Each to their own, when it comes to Tonality, come SUT's are overbearing with Richness of Tone, while others are as a comparison seemingly Transparent.

The HA is undoubtedly on the Transparent side the presentation, and I have had indelible experiences where the Jaw has been agape with wonderment with Tube Phon's. 

@viridian "MC phono cartridges produce very low output voltages compared to most of their MM/MC brethren. And phono stages amplify voltage. But most MC cartridges produce a goodly amount of current"

-there is a transimpedance amp for amplifying signal in the form of current, and delivering the output signal as a voltage, here is one implementation of it: 

https://www.stereophile.com/content/sutherland-engineering-little-loco-phono-preamplifier-measurements

Here is another way to look at this subject.  My best FL system uses a $9k cartridge > $5K SUT > $28K phono stage > ~ $800 in phono cables.  Here in NH (today) a $550 cartridge > $3K phone stage > ~$400 phono cable.  I enjoy the music just as much listening to either system.  The delta is even crazier if the entire systems are compared.  Now to mitigate in my defense, a) it has taken me decades to build up these systems, b) the numbers quoted are current retail, often I paid a bit less, c) I am retired have no other responsibilities any more, d) have no other vices.  With all that said clearly I'm insane.

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.