I hope this helps, I can’t recall who it was that asked the question... but if they are on here hopefully this helps... it’s always about timing and phase
Get that good and you can then often adjust a sub level and still get a good relationship at the listening position...
As an example I have two front subs and I have the phase and timing of the rear sub nailed... I can add 12db of bass at 20hz tapering down to 0db added around 80hz and it doesn’t pull back at all, only because the phase and timing at the listening positioning is nigh on as good as I can dial in... it drifts a bit at very low frequencys due to one being a 15 and one being a 10” (low frequencys often lag a little I think I have read somewhere more so with some drivers vs others...)
but at low frequencys due to wavelengths concerned it’s harder to get them in phase to a degree as the time has to be much larger and as long as you are within 60 degrees you still get a good amount of summation and don’t miss much...