SUT vs. Phono preamp


On what basis is  the decision to chose  a SUT vs. a phonostage made?
boofer
I have 5  LOMCS, one phono preamp and one SUT. Actually, I do not notice much difference between  the phono preamp and the SUT, but I just wanted to know a little more about the theory behind all this; your responses so far  have helped  greatly. 
@billwojo,

I had a Denon AU320 SUT teamed with my ZU-Denon DL103 and I didn’t particularly like the sound as it sounded kind of flat.  I sold it and bought a Bobs Devices SUT with the Red Cinemag transformers.  The sound was clearer and much more involving.  
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