Grimace,
I do not know exactly what was happening - my guess is that @ 4' from the wall behind the speakers, speaker must have been in a peak node of a bass standing wave (all rooms have standing waves & their freq differs based on their/room dimensions) which caused that freq to be enhanced creating a boominess.
I *assumed* that your ceiling height is 8' & calculated the standing waves for your room. The axial modes are most powerful. Axial modes involve floor-ceiling, both long walls, both short walls i.e. any 2 parallel surfaces. I found that the floor-ceiling standing wave freq is 70.625Hz. Using speed of sound = 1130ft/sec, the wavelength of 70.625Hz is 16'. So, a quarter wave of 70.625Hz = 4'. So, if your speaker was 4' from the wall behind the speaker, you must/could have been at the peak of the 70.625Hz wave & that bass freq got enhanced creating the boominess.
Also, a tube amp, in general, does not have low enough output impedance (just the nature of tubes) to control the bass well to begin with. So, any room impairments could easily overwhelm such an amplifier.