The studio had a slap echo you could clap into and count. For a year we hung things on the wall that looked like they should help and did not.
Everything we hung was chosen by eye and judged by ear. That mismatch was the whole problem.
What fixed it was cheap, ugly, and behind a curtain: mineral wool in the two corners, a rug with an underlay, and a bookshelf turned ninety degrees so the spines faced the room.
The bookshelf is a diffuser. It has always been a diffuser. We had simply never let it face the right way.