SUT Advice - Which Ones Are The Best?

I have four SUT’s: Bellari, Denon and two custom-builds using vintage mic transformers. Presently one of the custom jobs is paired with the Coral 777 Sleeping Beauty lomc on the FR29 arm/Ariston RD11 TT. All these SUT’s have there own "flavor" - they all sound "good"! The Bellari was bought new and was the most expensive. The Denon is in use with the Denon 103R on the Pioneer PLX1000 TT. Previously it was used with the Bellari. The other three came from eBay sellers and averaged ~$200 each. So no need to spend a lot of money! I like to have a variety to play with different lomc cartridges!

I use Hashimoto HM-3 and Cinemag 1251.
I am quite familiar with other SUT Brands and Head Amp's used within my own system and other owned systems. 
All my experiences I have taken part in will allow for me to detect that a  SUT will add a richness to a replay. Even though this trait is difficult to detect on certain Brands of SUT,  it is a noticeable trait when a SUT is A/B compared to a Head Amp'.
I have to date not heard a SUT that has a comparable transparency that a Head Amp' can produce.
In my system I am comfortable with each presentation and at present feel that the Cinemag 1251 has the Interface and Presentation that is most suited to my preference, it has a design that enables it to project the Upper Mid's and produce a very controlled Tight Bass Note.    

It is sometimes said that an SUT should be matched to the cartridge (or the other way around).
The reason typically given is the need to have an appropriate amount of SUT gain to bring the cartridge's output up to a level that is comfortable for the chosen phono stage.

However, there is another less-known but equally important reason, which is that the low frequency bandwidth and phase response of the SUT depend on both the inductance of the SUT's primary windings, and the net impedance that the primary windings will see, according to the following formula:
f = Z/(2*Pi*L)
where f is the frequency where the amplitude is 3dB down compared to higher frequencies,
L (in henries) is the inductance of the SUT's primary windings
Z (in ohms) is the net impedance

The inductance of the SUT's primary windings should be measured with the secondary windings left open. (Note that a completed SUT may contain internal load resistors across the secondary windings, therefore it is risky to assume that simply because the SUT is not connected to a phono stage, the secondary windings are truly open.)

"Net impedance" in this particular situation means the phono cartridge's source (self) impedance, in parallel with the impedance that is reflected back from the SUT's secondary windings (when connected to a suitable phono stage). Usually the net impedance will be dominated by the cartridge's source impedance.

The mathematical formula tells us that in order to improve the low-frequency capabilities our options are to lower the source impedance (choose a cartridge with less self-impedance), increase the primary inductance of the SUT (increase the number of transformer windings, increase the dimensions of the transformer core, or use a higher-permeability material for the transformer core).}

In a nutshell, the larger the self-impedance of a cartridge, the more primary inductance will be required from the transformer in order to keep the same low-frequency extension or phase response.

Conversely, if the cartridge has small self-impedance, less primary inductance will be needed for sufficient low-frequency extension or phase response.

Although the target frequency for "f" in the formula above could conceivably be set to the lowest frequency expected to be reproduced, if good phase linearity is likewise a goal, this will require a considerably lower target frequency.

Caveat: although good low-frequency performance from a transformer requires sufficient primary inductance, since high inductance (in the form of more turns, or more wire length per turn, or higher-permeability core materials) tends to pose problems for high-frequency performance, single-mindedly selecting a transformer with as much primary inductance as possible, isn't recommended either.

Summary: When choosing an SUT, be sure that the gain is appropriate for the output level your phono cartridge.
Also be sure that the SUT's primary inductance is suitable for the coil impedance of your cartridge.
When connecting a SUT and phono stage together, try to choose as low-capacitance cable as possible, using as short lengths as possible.
Generally SUTs work better with low-impedance cartridges.

@jcarr thanks for the detailed response. When you say low impedance what number are you looking for?

I forgot to add that I have a fifth SUT on hand: a pair of Sony ST10’s that are the smallest I’ve ever seen. They are square cross-section, about two inches long, an RCA plug at each end - one male and one female. They are each a single channel and plug directly into a phono stage’s RCA jacks. Amplification factor of 10X. No ground wires. Rather neat! Bought from an eBay seller for $175.