Master Handbook of Acoustics is your friend if you want to learn what to do to your room. Overkill for most, admittedly, but it contains everything you need to know.
Here’s the pdf to the 4th edition.
What improvements did you make based on the book? I’m skimming through it.
I made couple of bass tramps tuned to the room’s main resonant frequencies, which I measured. I followed instructions from the book.
I added sound absorber panels to the walls and ceiling to kill immediate reflections from the main speakers plus a sprinkling of additional panels to kill reflections and also act as decoration. I also needed to move one radiator because it was in the worst possible location for my setup.
The room got thick curtains to improve absorption, and they also darken the room as it is dual use music listening and home cinema room. A few defraction elements went into the ceiling for a good measure. The ceiling is made of custom panels that I made myself from wood and fabric to allow sound energy through to the various acoustic elements behind them.
I also spent a fair amount of time with subwoofer placement, but in the end it became a bit of a compromise between sound and placement of furniture. Nothing a bit of signal processing can’t deal with, mind.
