"Further, how could one separate cause and effect when trying to determine where the benefit is really coming from?"
By testing and changing variables in small increments.
The high SPL low SPL discussion is an odd one. Alot of good effects come from overdriving amps and speakers.
However you do not need to move air to get a good sound. A very light very small cone driven balls to the wall by a tiny tube amp that's glowing and humming sounds like a tiny little box in the room but shove a mic up in the grill and suddenly it (can) translate into something massive through the monitors. However a massive amp that has ear splitting mids and back melting bass just puts tons of energy into creating mud in the room... You record the mud and on playback the phase cancelation causes holes in the sound and (can) make for a thin translation...
How you arrive at a good tone takes time, no matter who you are. So you have to spend alot of time playing with different amps and different mic choices and positions.