When is it time to consider adding a SUT?

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.  

Dear @billstevenson : Well, that kind of designs exist in the market. I'm using one for some years now and outperforms not mid-level tube designs but any one .

@gtaphile :   "" not know what is missing until you try a SUT in the chain.  "

for " thousands " of years I used SUTs with either: tube and SS designs. I owned almost every SUT in the market. In those time I really did not know nothing of what " today " I learned/experienced on the SUT subject.
As a fact I still own Denon, Entré and Audiocraft SUTs. I still use it time to time, so I have past and today first hand experiences.

I can tell you that you can't miss anything through the SUT experiences against a today good active design using today bipolar devices. A SUT can't compete in anyway against today bipolar transistor. The audio signal is degraded at higher levels through a SUT than a transistor. Problem is that the audio signal in your system pass through tubes in the AR item.

Audiophiles have different kind of experiences and learn different " things " and sometimes exist misunderstood in what we learned.

Regards and enjoy the MUSIC not DISTORTIONS,
R.