[Andrew W]

The spreadsheet, good as it is, as supplied only calculates axial modes, which admittedly are the most powerful but tangential modes i.e. those that strike four surfaces as opposed to the two in axial modes, only lag around 3dB below but at very different frequencies due to the varying path lengths.

When I was teaching acoustics in a music college I used to impress on students that calculating modes was interesting in theory, but not much use in practical. There is no short cut to identifying "boom" frequencies but by lenghty actual measurement with a frequency generator/ or test tone CD and a sound level meter. When you have identified the one or sometimes two fundamentals which are causing problems and treated them by trapping you will be on the way to hearing your equipment/system as the manufacturer intended.

I also used to stress that the sound system is the equipment and the room.