So when is a SUT really needed?

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? 

The main thing you "need" one for is when you have a LOMC cartridge and your phono stage only has an MM option.  Everything else is subjective.

I recently dipped my toes into the world of SUTs for a specific reason.  I have two turntables and a lot of records.  One table has a very expensive (at least to me) cartridge and I'd like to use it for my best audiophile records only and extend its life. 

My Herron Audio VTPH-2A has one MM and one MC inputs. I've tried four different SUTs recently, using a ZU Denon DL-103 on the second table (a vintage JVC QL-Y66F) using the MM input and achieved sound quality that approaches the sound quality of my main table. 

Somewhat by chance, I tried one of them with my Hana SL, which up to that point I had found somewhat laid back and reserved, and was quite surprised at how the SUT transformed it. It sounded better than the Denon had and has more dynamics and detail than I remember hearing from it in the past.  The SUT I'm using is a Jena Labs / Echo Step It Up, which is the one of the four I'll be keeping.  My main table is still better, but I can be content playing the many less than perfect recordings I have, not to mention the convenience of the JVC being fully automatic. 

The only way you'll really know if a SUT is for you, is to try different things in your own system.  The great thing is that there are lots of pre-owned SUTs and you can buy one, try it, and flip it if it doesn't work in your system without losing much, or anything.