Fine tuning the system. Where do you start and how do you proceed ?

Most important thing for me is make ONE change at a time.  If you are trying to find the optimum speaker placement , focus on that, don't throw other variables into the mix.    

Same with a component change.   One change at a time. 

I don't use a "reference" recording.  I use a number of tracks.   Most are tracks that are deeply imprinted in my brain.    I think it's easier to notice where a recording shines on a system.   I also use genres I don't normally listen to so there's no bias as to what I'm hearing.    

The chase for sound that does not end for sure...But if what you have is the best you have ever had enjoy the sound and slowly upgrade...I was sort of lucky in some ways that I purchased first tube amp from Raven Audio and tried their speakers from cheapest to most expensive,,And was not happy until I had the Corvus speakers .And I have upgraded 3 times on their Amps and now have the Reflection and very happy with all the detail I hear from my equipment ..But yes but I know that there is still more to capture for me to hear... Move the speakers and if using sub move it around..I know my living room is no sound room but mostly I am happy and wife has to be out of house for me to really enjoy my system for just playing 74db she is not a fan of music...That is when I relax with my Tinnitus goes away into the music...I do not play anything other than down streaming and my best sound is Tidal and using Audio Quest Diamond which gave me the best sound I myself could hear out of 4 cables I tried..Enjoy the Hobby and wish you well in achieving your best quality sound...What are you using for your system amp / speakers etc ??

You seem to have no major issue with the current system.  What exactly do you think could be improved.

  My suggestion is to make sure that you have a comfy listening seat, and adult beverage of your choice, and play music with obsessing about what small differences in sound are possible 

Good suggestion....