Someone asked for some basics from Jim Smith:
First, determine the optimum listening position in the room by determining where bass response is best (most even, not at a null). Instrumentation can help here.
Next, relative to the listening spot, find the optimum speaker placement. Start at the 83%rule mentioned above, as a starting point. Adjust from there. Many fine adjustments over time are necessary for best results.
Treat first reflection points.
Lastly, a blend of absorption and diffraction is best ( ratio of 1:2 respectively - more diffraction).
All other things equal, avoid equipment between speakers. If unavoidable, place it low to floor and behind speakers.
Before spending on better equipment, try the above methods to improve performance of what gear is already there.
Harder to do than say, admittedly.