So when is a SUT really needed?

"I like how it sounds."  That about sums it up.  You use a SUT with an MM phono stage for your LOMC cartridge, if you prefer that result to using your LOMC with any other amplification device, which might include a higher gain phono stage that in and of itself provides sufficient gain or an outboard active gain stage ("head amplifier"), which for the OP's benefit is a plug-in device that adds only gain, not RIAA equalization, to the output of the LOMC. The output of the head amplifier then must be fed into an MM phono stage for RIAA correction and for added gain, just as in the case of using a SUT.  In recent years (to the OP), the idea of amplifying the current (not the voltage) output of an LOMC has come into vogue among some.  So now you have the additional choices of using an outboard gain stage that operates on current gain and that then puts out voltage to drive an MM stage or a current-driven phono stage that affords lots of voltage gain at its output sufficent to drive the downstream linestage or direct to an amplifier.  It's a bit bewildering, and there is no perfect answer. You have to try a few of the alternatives for yourself and make your own decision. Moreover, one doesn't know what else you (OP) do or don't know, so please ask more questions.

It really gets down to trying different things.  In my years of doing this I have acquired MM, MC, MI, low output, high output, (I don't even know how many cartridges I have) two phono stages one very expensive tube one, and one moderately expensive solid state one, two SUTs, one silver wire and one copper.  I can't tell you what combination is "best" depends on the record, my mood I suppose.  Elliott correctly pointed out to me in a previous post that people just have to try things.  The truth of the matter is that I am happy as a clam with a MM and always have been.  My absolute best is a LOMI > SUT > tube phono stage.  But I am just as satisfied listening to a recently acquired MM > SS phono stage that cost one twentieth as much.  The thing is, though, in order to gain that perspective you have to experience different things.  There is no substitute for experience.

I owned firsT Klyne 7PX3.5 phono amp but wanted one suitble for two arms.

So  I moved on to ASR Basis Exclusive with ''two in one''. There was also

another difference. Klyne had ''only'' 4 amplification stages Basis 6.

but both  recomended the lowest amplification related to the given cart.

My ''reduction of complexity'' was :'' the higher the ampilfication the higher

distortions''; so nether recommended the highet amplification. 

If this can be seen as ''valid argument '' why not even Lew with his 

theoreticsl reasoning about carts wich he never owned deed not react?

@lewm 

"Faustuss, I think you need to look up “diatribe”."

Mirriam Webster lists it as synonymous with "a prolonged discourse" hence the context as in going on and on and on! Get it?

Funny, the choice you made for a definition of "diatribe".  Here is mine, also from Merriam Webster:

https://www.merriam-webster.com/dictionary/diatribe

Your definition for diatribe as a "prolonged discourse" is mentioned as a third option and noted to be "archaic" in Webster’s.  I delivered no diatribe by the modern definition in Webster’s (see above) or as defined in the Oxford English dictionary, for that matter. Meantime, don’t you think this argument is off topic?

Nandric, after more than a decade, you might have learned to spell the past tense of the verb "to do".  It’s "did", not "deed".  But what indeed did I do that brought me to your attention?