I was lucky enough to have a friend whose profession is building home theatre spaces, and he owns many thousands of dollars worth of audio instrumentation and software and he was kind enough to help me set up my combination listening room/home theatre.
I already had a false curved ceiling which helped on a couple of issues, and the one live wall made up of glass doors was happily handled by installing heavy optically opaque curtains which I needed for video anyway.
Many of the other issues were dealt with inexpensively. For instance two and a bit walls lined floor to ceiling with shelves of vinyl LPs make a great diffuser (and no, I did not stow them with every other album pulled out or pushed in although it would theoretically have enhanced diffusion.
Speaker set up was a mix of a ballpark area they needed to be in plus accurate measurement - me in a listening chair, him with a laser range finder to accurately position my chair and the speakers. Ditto for the four surround speakers and two subwoofers.
Undesirable echos were dealt with by things as simple as laying an antique carpet on the too live wood flooring.