So when is a SUT really needed?

@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!

What’s a “SUP”?

Longer explanation of SUT to follow when I get out of bed.

OP: "So when is a SUT really needed?" Ignoring Esoteric Designs:

When, in the natural progression of things, 5 below, you finally try a LOMC Cartridge (Low Output Moving Coil).

MC theory: precisely controlling the movement of a smaller/lighter mass, the coil, can be better than controlling the movement of a larger/heavier mass, the magnet). The smaller/lighter the coil, the lower the output signal strength.

Makes Undeniable Sense, thus Eventually you must hear LOMC for yourself. 

1st you get a TT with a Moving Magnet Cartridge, probably elliptical stylus shape, more than likely aluminum cantilever.

2. You need a MM Phono Stage to both boost the signal and perform reverse RIAA Equalization that occurred during recording, which boosts the highs to minimize noise and cuts the lows to reduce the bass wave lengths to reduce the wide bass groove widths, to get more music on the 12" disc, which, because of the benefits of this scheme was designated ’Long Play’.

3. you learn about advanced stylus shapes, and stiffer/lighter cantilever materials, move up the chain of MM cartridges, perhaps buy used, perhaps get involved in re-tip/re-build/upgrade worn or broken cartridges.

4. you ’improve’ your MM Phono stage. All my preferred choices have been Tube MM Phono Inputs, either in a vintage receiver; preamp; integrated; or separate MM phono stage.

5, Finally Try a LOMC Cartridge. They have ____? signal strength; and their coils have _____? Impedance

6. Learn about Step-Up Transformers, because MM Phono EQ Gain is not enough for LOMC weaker output signal strength.

7. Learn about Impedance (Ohms), because while SUTs are Passive, and might include optional windings to produce optional voltage gains, they also passively/resultantly effect the impedance that the cartridge’s coil will be loaded by. Each gain option effects the Impedance Load differently. 

8. Impedance Guidance Formula: Coil Impedance ____ Ohms x 10.

A LOMC cartridge with a coil impedance of 10 Ohms would have a recommendation for Impedance Load of 100.

Many (I’m tempted to say most) simple/basic MM/MC Phono stages, the MC impedance load is 100 ohms. Specs, often found in the device's manual, should give the Gain and Impedance answer(s), as well as minimum signal sensitivity and overload.

Coil of 12 Ohms ....

My chosen SUT and some info I gathered

https://www.audiogon.com/systems/11318

Elliott, that is all quite true and correct and pretty accurately reflects the path that probably most of us followed.  The realization dawned on me, though, that following this path is not necessary.  The truth of the matter is that a good MM is good enough for all practical purposes.  Or a HOMI.