SUT into MM input section of Solid State Phono Stage

What is your MC cartridge then? Start from the cartridge, not from the SUT. If you already have SUT then you are limited to certain type of LOMC according to cartridge impedance matched to your SUT. You can't use one SUT for all LOMC cartridges. 

+1, chakster. In most cases, a customized SUT for your MC cart should sound better than your internal phono’s MC step up transformer.

That IS the purpose of a SUT that is properly matched to the LOMC.
Or am I missing something?

Dear @no_regrets : what you are experienced is something that happens to often and depends mainly of the quality level design of the active high gain stage in the phono stages and not because it's better the SUT/MM combination.

Btw, what other gentlemans as @lalitk and @fuzztone posted here is not exactly that way because it's not the cartridge and the SUT impedance what must be matched this is totally false.

A LOMC cartridge is not affected by SUT impedance, it's insensible to loading impedence changes.

   " Loading the transformer does have an effect on the load seen by the cartridge, but any change in sound quality is more likely to be due to altering the performance of the transformer, not the cartridge . "

What must be matched between a SUT and the cartridge is the SUT gain to the cartridge output level, this is the main issue and not impedance loading.Any SUT with the rigth gain can works with any cartridge with the adequated output level it does not matters its impedances.

Low knowledge levels in that subject makes that audiophiles post wrong information/advises.

So, just continue listening where you are satisfied.

Regards and enjoy the MUSIC NOT DISTORTIONS,
R.