Christian motivated me to re-examine the switch story.
I kept as a reference the sound where the K50 plays without Ethernet from SSD (no rj45 connected).
First, replace my current fiber connection (with media converters) with CAT cable (audioquest).
This resulted in deterioration in sound quality.
Then Media converter on the K50 side replaced by switch with SFP input.
No improvement.
Switch with SFP powered with by a LPS (12Vdc diy).
Big step forward.
Since I still had another switch lying around, I connected it between the switch and the K50.
This step was very big but the sound became harsh.
After i powered this switch also with a LPS i get a great result (the holy grail).
Note that I obtain this result in my situation so no guarantee.