Trial and error. Big problem. It’s not really science. What’s required in order to have a glimmer of hope of finding the absolute best solution for room treatment is to employ a SPL meter and test tone generator such as 315 Hz, for example, on a test record or CD. That way you can precisely locate the problems in the room, a great many. The primary acoustic anomalies in the room are sound pressure peaks more than 6 dB above the average sound pressure in the room - e.g., room corners, standing waves, first and second reflection points, sound pressure peaks behind the speakers, peaks in the 3D space of the room.
Room echos can be easily identified using wood blocks and walking around the room clacking the blocks together or clapping your hands 👏 until you hear the distinctive echo. Once you have treated the room, use the speaker placement track on the Test CD or LP to find the absolute best speaker locations. If you add more room treatment at a later date, tiny little bowls, Mpingo discs, whatever, you need to reestablish speaker positions as room dynamics will change. But, gentle readers, trial and error for room treatment - like its cousin speaker placement - is like trying to solve X number of simultaneous equations in X + N unknowns.
