When is it time to consider adding a SUT?

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.

Dear @taxmandme : """  Also, without the SUT in the system, I could never hear any difference by changing the load. """

That only confirm that LOMC cartridge are " non-sensitive " to impedance load changes and confirm that in this regards your AR design is a good one.
What happens when you introduce a SUT in your hybrid PS is that exist a reaction inside the PS circuit wi9th impedance/inductance and capacitance and this is a real problem and not desired for the cartridge can shows it at its best.

Good SUTs sounds good but has its own limitations on noise and distortion levels and of course limited frequency range with out deviations where a good bipolar full SS phono stage can outperform your today set up.

Regards and enjoy the MUSIC not DISTORTIONS,
R.

There is only one problem with your analysis: "the bipolar full SS phono stage" that sounds better than even a mid-level tube phono stage is unobtainium.  Nice try though.

So we now have many posts quoting the science and others quoting user experience indicating that a quality SUT and cables may in fact improve the end to end performance of a high quality and capable PS even if they have sufficient gain required for LOMCs.
I am still waiting for my cartridge to come in and I will listen carefully for any anomalies even though one might not know what is missing until you try a SUT in the chain.