So when is a SUT really needed?

Making a reasonably quiet MM tube phono stage is fairly easy, but making one with 60 db of gain is a supreme challenge.

I have found the NVO (New Valve order out of Cyprus whcih uses either 14 or 22 tubes) is quieter than several / most SS phono units inclucidng the Parasound JC3+ Phono. It's all about the implementation. As @viridian  also stated it's all about prefrences and to me SUTs are the way to go. It was actually Kondo-san, the owner of Audio Note Japan that was my mentor and taught me all about SUTs. In his line of equipmewnt (he designed the original Ongaku amp) he only made MM phono stages and his cartrigdes had extremenly low outputs, something like 0.15mV. It was a must to use his SUT with his MM stage to get the right vol out of the system. Over my last 25-30+ years I have played with dozens of SUTs and recently had one made from Consolidated Audio out of Germany that perfectly matches my MSL Sig Diamond cartridge. Yes a match made in heaven that I now can hear the angles sing. (Maybe I'm just getting old and those angles are closer to me than I hope). So all I can say is for the OP to borrow a few SUTs and hear what they bring to the table. 

Best of luch in your own journey. 

Hats off to @viridian for explaining the question very well.  No one needs an SUT, tube or SS, as @lewm explains.  At the moment, up in our NH home, I am happily listening without the benefit of an SUT.  When we go back to our home in FL in a few more weeks I will almost always prefer listening with an SUT.  This is because of available equipment, up here in the North Country I am deliberately keeping things simple.  If I were not there would usually be an SUT here as well.  You really need to try listening to a few SUTs as every one is different in order to determine your own preference.  BTW, you would need an extra phono cable identical to the one you use already.  Of course this adds to the expense and the potential for noise and line loss so the cables should be kept reasonably short.  Obviously the incorporation of an SUT into your system should not be taken on lightly.

Thanks for all the feedback, this helps a lot.

So far, my understanding is that, technically, if your going to use a SUT, then the phono preamp doesn't really need MC coil adjustments, correct?

But on the other hand, most will want to choose a good quality phono (Tubes or SS) and it will probably have MC adjustments anyway but they won't be utilized since the adjustments will be done through the SUT.

I guess, I am wondering is using a good quality SUT better than using a good quality Phono by itself?

I have Two readily available Phon' options to not use a SUT or a Head Amp, and my Third Option Phon' to not use the SUT or HA is ready for myself to pick up.

I also have a MM Phon' that requires a SUT or HA for my MC Cartridges.

I am always Wed to the SUT or HA as the method to use the available Phon's and am keen to learn how the New Phon' will be an influence in the home system. I have heard multiple hours of a version of this Phon' through the dedicated MC Stage and can assure I have been seriously impressed. My own system is the next challenge to have the experience.

SUT's and HA's are for me, able to inject a variety of Tonal Qualities that I am attracted to as the End Sound. For this reason I like more than one influence and mix it up across different SUT > Phon's  + Different Phon's with a HA, where thye remain in use as a certain configuration for quite a few hours before a change is made.

Only occasionally does the dedicated MC Stage get used for an extended period, usually a few Tracks is enough to want to return to the SUT > MM Stage.

Each to their own, when it comes to Tonality, come SUT's are overbearing with Richness of Tone, while others are as a comparison seemingly Transparent.

The HA is undoubtedly on the Transparent side the presentation, and I have had indelible experiences where the Jaw has been agape with wonderment with Tube Phon's.