I did some THD measurements of the in-floor subs using REW and calibrated UMIC. The readings were taken in the Listening Position which is 12ft away from either sub.
The room is open concept, 45ft deep, 28ft wide, 10ft ceilings. Open to both basement and second floor through stairs. No treatments on walls, lots of hard surfaces, very little furniture, lots of windows. There is a horizontal bass trap in the back upper corner.
The normal listening level peak is 105dB Pink Noise (2Hz-20Hz noise spectrum) in Listening Position.
That is the max level before LF starts rattling the windows, patio door, floor, walls.
Any louder than that and it gets too much LF and the room can not take it.
There is no EQ on the subs.
The subs are xovered at 40Hz.
The subs were drawing no more than 400W each side (-20dB LED on CROWN XLS1500 is solid on; -10dB LED never came on; subs drivers wired for 8 Ohm)
I measured THD starting with the Noise Floor and then continued with the 8Hz - 37Hz band.
I knew the subs were clean sounding even before I started.
The THD measurements really surprised me though. THD% was around 1% anywhere in the 8Hz-37Hz band.
That is a mid-bass driver THD territory, not a subwoofer THD territory.
That shows the advantages of using multiple smaller subs vs one single sub even when the displacement is equal.
Some of the advantages are increased sensitivity (add extra +3dB each time number of subs is doubled; here that means 9dB more sensitivity - which means the total power for all 8 subs is 1/8 of the power needed for 1 large sub if big and small driver sensitivities are equal), reduced thermal compression (the 1/8 power is further dived between the individual drivers - in my case that's about 40W/driver; heat dissipation is a lot easier when the heat load is spread out to multiple drivers) , reduced THD.