SteveC, this I understand but it doesn't explain the room size part.
Sure it does. I mentioned room gain - the smaller the room, the earlier the onset of room gain, the more "free" output one gets. I mentioned boundary gain - the smaller the room, the more likely the sub is going to interact with boundaries, the more "free" output one gets.
Without a seating distance the comparisons are meaningless.
Seating distance should be included, but in regards to low end output benefitting from room gain, or overall output benefitting form boundary gain, room size should also be included. If Steve nn can get those 112db peaks at say 14hz in his wide open room, that says something, cause he isn't getting as much room gain as someone with a much smaller, closed off room.
Let's look at the Rythmic sealed 12" Servo unit. Crackyflipside has one in a corner of his 7'x10'x8' room, it is setup to be relatively flat to 10hz, and he is getting sufficient output. Crackyflipside then paid Chuck a visit and brought his sub - Chuck's listening room is 16'x24'x8' AND open to other rooms in his house. After setting it up to be flat to 10hz in a corner, and trying to do an 85db sweep from 9' away (which is pretty typical), it bottomed. Now distance to the seat in both of these circumstances is needed to make a valuable comparison, and I don't know cracky's, but let's assume an extreme case and say only 3'. A typical room isn't an anechoic chamber, so we'll assume 4db loss every doubling of distance, giving cracky 6db more headroom due to seating distance.
Now, 6db difference alone isn't going to make much of a difference with bottoming occuring at 85db. 91db isn't very loud for subwoofers. What's happening is that room gain is setting in much earlier (at a higher frequency) in cracky's room than Chuck's, and if we assume typical room gain to be 4-8db/octave, getting the onset of room gain at what I think calculates to be 56hz in cracky's room will play a VERY big role. Chuck's room, not taking into account the fact that it is open to other rooms in his house, wouldn't start seeing room gain until around 23hz - considering it is open to other rooms, that onset point will probably drop down into the teens. So even assuming worst case scenario of only 4db/octave of room gain, cracky is gonna have at least ~7db more gain in the teens than Chuck. Realistically, it's probably gonna be more like ~14db.
Finally, if a room was infinitely large, it would behave like an anechoic chamber, with NO room gain and little to no boundary gain - that definitely affects output levels seen at the seat.