When is it time to consider adding a SUT?

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.

gtaphile

As a bit of a simplification a MC phono stage does two things.  First there is some gain, and secondly there is equalisation  - usually RIAA.  That first step can be done

1. On the cartridge (High Output Moving Coil);
   
2. Electrically - MC Phono Stage
3. Externally via a Head Amp
4. Externally via a Step Up Transformer.

This latter option is not adding a component.  Yes you see another box but the Transformer is a passive device (the actual key to why they are so effective, by the way).  If anything it is subtracting a component as you bypass electronic circuitry.  Other things being equal and the Transformer is well done it is the perfect solution.  

Other things are not equal and the downsides to an SUT are that they are susceptible to noise if placed too close to other components such as power supplies.  They are expensive, the good ones anyway.  Also there is the issue of impedance matching.  If the quality of your Phono Stage is good, and I guess that includes ARC, then I would not bother.  On the other hand I have read in other forums of of good results with Step Ups into other high quality phono stages.  One of the best resources out there is by Andrew Rothwell

http://www.rothwellaudioproducts.co.uk/html/mc_step-up_transformers_explai.html

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.

gtaphile, FWIW, I'm using an ARC Ref 2SE with a Lyra Skala cartridge (0.5mv) and while there is enough gain using the high gain setting, I am much happier since I've added an Ortofon Verto SUT.   With the SUT, the dynamics, detail and headroom are immense compared to the sound without the SUT.   Also, without the SUT in the system, I could never hear any difference by changing the load.  Now there are significant differences by varying the load.  Just my 2 cents.  

I agree with taxmandme, my phone stage is a Conrad Johnson TEA2MAX, which has 55 db of gain.  This is a very high quality tube phono stage.  Although it can handle a range of cartridges from 0.3 mV to 5.0 mV, for any of my low output MC cartridges it is much better to use my recently acquired Bob's Devices Sky 10, set for 5:1 step up.  Much better.  There is a sense of ease about it, greater dynamic range especially noticeable during the quieter passages of performance.  As others have pointed out adding an SUT requires compromise.  In addition to the SUT, a very good set of interconnects is essential, so this is not a trivial tweek.