So when is a SUT really needed?

RB, Sorry for the 2 day delay in posting this comment, but I have been busy.  I also apologize for seeming to harp on your post of Feb 17, but there are a few issues with which I take serious issue, and since we think we are talking to the OP as if he is a neophyte, I thought it was worthwhile to set the record straight, at least as I see it.  You wrote the following:

"In general, there is a trade-off between output and trackability - the ability to precisely follow the groove.  For good trackability, you want low effective tip mass, which means smaller parts and less output.

There's another subtle consideration.  MM and MC cartridges must generate electricity, which reduces trackability as generating power electrically 'stiffens' the stylus."

Yes, it's true that the average LOMC cartridge (low output example) exhibits low compliance, but it is compliance along with a few other related parameters, not output, that determines the ability to track the groove. For example, there are a few LOMC cartridges with high-ish compliance (perhaps not as high compliance as anyone's favorite MM), and they do track well.  You went on to say that there is a correlation between tip mass and output, but in fact among electromagnetic cartridges (to exclude optical cartridges), the type with lowest tip mass is moving iron. And high output MI cartridges can have very high compliance. Furthermore, MM cartridges have the highest output among electromagnetic cartridges and on average track best. Finally, as far as I know, what is said to "stiffen the stylus" in the case of an LOMC is when they are driving a lowish input impedance.  This is sometimes given as a reason to use a 47K ohm load even with LOMC cartridges.  Furthermore, some gurus with expertise beyond mine doubt the importance of the phenomenon entirely. I think you got some of the foregoing ideas from the DS Audio website, wherein they may be drumming up reasons for a optical cartridge, but maybe one should not have to swallow the whole enchilada.  If it will sound good, it is good.

@lewm 

No worries.  I did not want to muddy the waters by introducing too many cartridge technologies, so I deliberately left out moving iron (and strain gauge!).

I just wanted to set out when a SUT might be needed because of very low output voltages.  (SUT not SUP as I erroneously wrote, though Stand-Up Paddleboards also have limited use as practical transport).

My comment on the need to generate electricity 'stiffening' MM and MC cartridges came from the thread titled What innovative, unconventional cartridge designs can you recommend? Audiogon Discussion Forum

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!