Speakers or room

select-hifi, talking of which, when I've been comparing two different audio receivers ( from this website: https://chooserator.com/compare-av-receivers ) models called  Marantz SR7011  and  Yamaha RX-A3070BL  -  I've chosen the most functional, which in my opinion Yamaha is. It can demonstrate really powerful and dynamic, but at the same time soft sound.

Hi Brian -

Your KEFs are too small for the room.  

My living room is about the same size as yours and speakers that play very well in the smaller second bedroom (10 x 15) that we use as a den, sound awful in the living room. 

You need bigger speakers with bigger woofers that take control of the space.  I am using now Zu Audio DWs (Dirty Weekend) and they work very well in the space.  My wife has complimented their sound and she is not one to comment one way or the other. 

Not certain what is available where you live (the Portugal is throwing me a bit).  But start with floorstanders with 8 inch woofers, that have some oomph/ pushing power to them.

Rich  

First cut off the bass from the LS50W and use one or two small dedicated subs with good room correction software. Martin Logan has new interesting subs with ARC room correction. The LS50W are fine speakers but don't let them play bass below 100hz. Just destroys the music. Good advice treating the room. 

I certainly would leave any room in which those speakers were playing.
/You are describing a digital amp. Try class A/B with speakers from a manufacturer targeting critical listeners.

I have a similar situation.  I've done the best I can without compromising my room appearance too much but I know it's not ideal.  Much of my listening is background so I'm ok.  Jim Smith's book,  "Get Better Sound"  will help you a lot.  Most of his info is about room set up.  IMHO, get advice here, take your time and tweak as you can.  Good luck.   Doug