yes you will get a boost from a second sub but your question was if it would go from a large to an extreme rating. realistically to achieve that you would need to stack them.
how much amp power you need as a minimum will come down to your listening distance from the speakers and the real world sensitivity of those speakers. with such a large room it's not unreasonable based on the speakers you choose, and your distance from them, that the sound pressure level at your MLP could be as low as 80 dB at 1 watt - this is worst case scenario assuming as much as a 16 ft / 5m distance from the speakers and those speakers being below average sensitivity. 86 dB at the MLP would be more typical for smaller spaces. even if you only ever listen at moderate levels you will still want to be able to achieve at least 96 dB at your seated position / MLP.
back to your original question of potential room gain. this could be relatively straight forward or quite difficult to figure out. simple fast rule is that you have a large space so there will be very little. a small room with a longest wall length of 14 ft / 4.25m should see a 12 dB/octave boost beginning at about 40 hz for a sealed sub (i.e. +12 dB at 20 hz). the gain is less for ported subs because they are rolling off naturally at 24 dB/octave and will likely be implementing a steeper subsonic filter to protect the driver from over excursion. if your longest room dimension is say 25 ft / 7.6 m then room gain won't begin until 22.5 hz. just to further confuse all of this you could experience room modes that can excite or cut certain frequencies along the path and are the primary reason why you should be very selective about subwoofer location when a single sub is used.
It's an open space with kitchen etc, so while large, the listening distance is actually 2.3m from the mains, 3.4 from the surrounds, and about 3m from the subwoofer.
Even if I don't change anything because other things are more important, I still want to understand how to measure the maximum of my system so I don't cross it. It's not clear to me if there is a way to get that kind of information from REW measurements (like find the SPL where distortion increases? Is that a practical approach?).
Subwoofer frequency response shows noticeable higher energy at the lower frequencies, and while it could be modes, I doubt that is the case for all frequencies past the ~50Hz peak, especially that I couldn't detect any obvious modes at most of those frequencies when playing a test tone. Same goes for the 30-40Hz range which doesn't seem to be boosted by modes. The 20Hz and 26Hz are modes to the width (20Hz, and actually 45Hz dip seems to be the 2nd harmonic even though it isn't exactly double the frequency) and length (26Hz) of the room. Room is about 6.5x8.5x2.57m (only ceiling measurement is accurate, and only 2 walls are 100% straight so it's not really a perfect rectangle).
Actually both 24Hz dip and 26Hz peak seem to be the 1st length mode, except at 24Hz the null is wide enough to affect the listening position (which is a bit away from the center of the null), while at 26Hz the null is narrow and the listening position is already getting loud (not as loud as the 2 walls, but louder than other frequencies).
Basically seems like before anything I should probably get a better receiver with XT32 or better. And probably convince the wife to get some minimal acoustic treatments stylish enough to fit in the living room. But I wonder if there are any other good value for money upgrades that will actually help (which requires understanding the limits of the current system, so that I can prioritize...).