There's no mystery in TN's methodology. He sets up in the comfort of his listening space, corner loads the sub and sets up his mic 2 meters away.
I've never been a fan of TN's results, as they're only comparable to his own tests of other subs and not repeatable except by him, but...
If you look at FabFrankie's compilation
of the subs TN has tested, updated through January of '06, the criteria is explained. You can insert the F112 by the numbers.
High harmonic distortion down low is usually perceived as tighter, more percussive subjectively by the listening comparos I've run of a sub with high HD at 20-30Hz vs a very low HD sub.
TN said that output isn't limited at 25Hz, as he recorded 104dB, and I would venture a guess that it would only be noticeable as having more impact to a subjective listening session playing soundtracks, without the THD numbers to explain why.
With the music selections, there isn't much below 40Hz that would show itself as high THD either way.
In the 1st 2 octaves HD is of little consequence and actually adds to the experience, IMHO, as most of it is subsonic, but from 20-30Hz, high THD is definitely audible. Whether or not it's offensive or pleasing is in the eye of the beholder as many 2 channel tube amp buffs and 2" analog tape recordists will attest. Those recordists actually play THD like an instrument, something subwoofer purists would (and have)scoff at. I keep an open mind where THD is concerned. Listening is the best answer to the THD numbers in any case.
Still, Ilkka's early SC and IMD tests indicate that the more offensive distortions follow THD numbers closely, so...?
It will be interesting to see the results of the investigation and the final results vs the Onix Rocket UFW-12 (both versions), Velo's DD-12 and the new SVS sealed 1X12" sub. Although I'm no fan of TN's stuff, I doubt there will be a flaw found in the methodology.
I don't expect much from a sealed 1X12, as I've built and tested many versions, and I'm really looking forward to the numbers of the F-113 and G-213 as they have more of what it takes, IMHO. Early subjectives from new owners of the 113 are very good. It will be interesting to 'see' what they're hearing