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

Assuming that you already have your "reference" copy of a recording, my next step would be interconnects and/or speaker cables.

In a perfect world, you'd have a few sets to try/listen to/compare. 

And, for the record, it's not always the most expensive ones that sound best. You have to find what works best for you in YOUR system.

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.