When is it time to consider adding a SUT?

Raul and I are more often than not on opposite sides of controversies like this one.  However, in this case, we are on the same page. On the other hand, I could well imagine that a tube phono stage with marginal gain when matched to a particular LOMC cartridge would sound better if a SUT is interposed between the two. That's no surprise to me, either.  So, this could also be a case where both sides are "right", because no one has defined the terms of the argument. How low does the output of the cartridge have to be in relation to the gain of the phono stage before noise and reduced dynamics become obtrusive, whereupon things get better if you add a SUT?  To some degree, that is a subjective judgement, unless we are talking about ridiculous mismatches.  However, in my opinion, the particular example posed by the OP should not benefit much from adding a SUT.

I use a bipolar transistor input stage at the front end of an otherwise all-tube phono stage. Raul doesn't like this, because it uses mostly tubes, but the tremendous gain afforded by the transistor does result in an easy, dynamic, unstrained, and very low noise (not to say "no noise" that I can perceive) presentation that was not obtainable prior to introduction of the solid state input device.  In fact, there is no LOMC that my preamp cannot handle with ease, up to and including an Ortofon MC2000 with an output of 0.05 mV.  But again, this is my subjective opinion.

andrei, I have to disagree with your statement that adding a SUT is not equivalent to adding another component, although you do make a good point that phono stages with enough gain for a low output MC usually use an internal gain stage to boost the output of the MC, which in effect is ALSO an added component.  (However, many phono stages with hi gain achieve high gain by the choice of the gain device [tube or transistor] used in the input voltage amplifier.  MM phono stages also add gain of necessity.  So sometimes it's just a matter of designing the amplification stage to suit one class of cartridges or the other.)  SUTs are not flawless, and adding a SUT also requires adding an additional pair of Interconnects and at least a pair of connectors in the signal path.  You might say that a SUT adds a different kind of problem, as compared to using a high gain tube or transistor to boost the low voltage signal of an LOMC.  Anyway, we agree that the OP does not need a SUT. Nor does he need a new phono stage.

I agree with audio_d.  Why add a component, along with ICs and connectors, in the signal path of a cartridge with a very low output voltage, when that is completely unnecessary?  But should you insist upon doing it, you would run the output from the SUT into the MM or low phono gain inputs of your ARC phono stage, assuming it has provision for high output cartridges.  In other words, if you use a SUT, you treat the LOMC cartridge as if it was an HOMM type.  Thus, you would bypass the FET gain stage that others have mentioned.  Not only do you not need to use a SUT, but also if you do insert a SUT, then you need to concern yourself with the quality of the SUT itself, a whole new can of worms.  Not all SUTs sound alike, let alone "good".  Furthermore, the high-ish internal resistance of the Benz is another good reason not to do this, or at least not to assume you will like what you hear.  Highs will likely be rolled off due to the impedance mismatch, which is not a coloration you want to impose on a Benz.