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

Fine tuning for me is not necessarily linear path, always about determining the weakest link. Not being afraid to experiment and fail is required, weakest links may not always be obvious. In the end everything matters, sometimes the smallest changes can make all the difference, tuning is an art rather than science.

Tuning by source material is problematic IMO. I have never used a small number of reference recordings to tune my system since this not how I listen. I play all manner or recordings, all genres, all qualities over the long term. If there are shortcomings you'll hear them in time, no reason at all to speed up the process, you'll only go in circles with short term listening with a certain number of reference recordings. Having patience and quit always listening with analytical part of brain is key. Enjoy what you have, the small displeasures may expose themselves over time, only then do you make a change. And never forget one change at a time, too many variables will only confuse.