So when is a SUT really needed?

In my post, for “tip mass” I should have stipulated “moving mass”, which I think is what you meant by tip mass.  The basis for the idea that low phono input Z stiffens the cantilever is Lenz’s Law which describes the opposing force that develops as a result of current flow. The lower the input Z relative to the cartridge internal Z, the greater the opposing force because current flow goes up. At least that’s how I remember it without internet consultation. Some more erudite types even claim that damping effect can be a good thing.

@lewm 

"In my post, for “tip mass” I should have stipulated “moving mass”, which I think is what you meant by tip mass.  The basis for the idea that low phono input Z stiffens the cantilever is Lenz’s Law which describes the opposing force that develops as a result of current flow. The lower the input Z relative to the cartridge internal Z, the greater the opposing force because current flow goes up. At least that’s how I remember it without internet consultation. Some more erudite types even claim that damping effect can be a good thing."

Considering your response to Richard's comment about tip mass and the above diatribe regarding electrical damping. Objectively, wouldn't you have already considered a cartridge that has a stylus with a nude mounting and an advanced stylus profile and the cartridge's electrical damping would already be part of the manufacturer's compliance specification; thus, a more important consideration would be matching the cartridge's specified compliance and weight to that of your tonearm's mass and compliance? Wouldn't achieving as close to optimum lateral and vertical resonance of 10Hz be as equally important? It has as much to do with trackability as the stylus' tip mass or as you are now saying moving mass? 

@elliottbnewcombjr +1 - your post describes well my own MMvsMC cart => SQ discoveries! 

@jay73 Stated

"But if you already have a Phono Preamp with MC adjustments, do you still need a SUT or is it used more in the situation where you may only have a MM preamp and want to use MC Carts?"

"Also, are all SUT's passive equipment, meaning they do not utilize a power plug or are there some that do?"

"Would there be reasons to use a SUT if you already have a good phono preamp with all the same adjustment options?"

To the above three inquiries I would cast the net wider than what is offered within this thread.

Now you have referenced the idea of using a Valve Phon' of which some option will lead to considering a SUT or Head Amp, I would definitely cast the net wider.

Posts from @mulveling and the communication that comes when this member has posted, will definitely broaden your knowledge base and remain in keeping with both items you are expressing a curiousness for.

What really matters is the End Sound, that is what is to be lived with and hopefully produced to a quality that meets your own personal and unique preference.

Their is a huge chance that a mathematical equation that will give you this End Sound that is for yourself assessed as being extremely attractive. Many many items as a sound assessment are rejected as not being attractive as a sound. All have a mathematical equation before them and a End Sound that follows.   

Footwork and experiences of End Sound for myself has always been the failsafe methodology to discover what is very attractive.  

Faustuss, You wrote, "Considering your response to Richard's comment about tip mass and the above diatribe regarding electrical damping. Objectively, wouldn't you have already considered a cartridge that has a stylus with a nude mounting and an advanced stylus profile and the cartridge's electrical damping would already be part of the manufacturer's compliance specification; thus, a more important consideration would be matching the cartridge's specified compliance and weight to that of your tonearm's mass and compliance? Wouldn't achieving as close to optimum lateral and vertical resonance of 10Hz be as equally important? It has as much to do with trackability as the stylus' tip mass or as you are now saying moving mass?"

I described my interpretation of RB's statement about the damping effect.  RB did not explain the phenomenon; he only mentioned it.  Based on my definition of a "diatribe", I would not say that my post contained a diatribe against anything. As to the rest of what you wrote, I don't follow you.  No matter what LOMC you are using, you have a choice regarding input impedance of the phono stage or whatever the LOMC is directly driving ahead of the phono inputs.  That choice is in addition to all the other elements that you mention.  Furthermore, I am not saying that cantilever damping as a result of current through the coils of an LOMC is good or bad; it's just an idea that has been discussed here and elsewhere.  Like I did mention, some think it is a problem while some others think it might actually benefit performance by the damping effect. Dave Slagle might want to comment; he does this stuff for a living.