I'll try to touch on a couple of themes in this thread that I think I can help clarify. Too many to try to quote...
1) The THX specification has changed over time. As mentioned, lots of good info in the HometheaterHiFi article. Before DD discrete audio, the subwoofer only needed to meet 105dB @ 30Hz. Presumably, there's also a distortion component to this spec, but I've never seen it published. Some time after the arrival of DD with it's +10dB boost in the LFE decoder, the goal posts were moved to 115dB. Either at the same time, or later, the f3 point was moved to 20Hz. As someone mentioned, THX does allow for some "standard" room provided gain. And, as shown in the Jamo manual, many subwoofers require multiples to meet the spec, especially after the 115/20Hz change. I remember back in the day (early/mid '90s), McIntosh's THX sub actually had a factory published anechoic f3 of 37Hz...THX has always considered room gain in the equation.
2) Bass, when you did your first video showing -20 for the test signal, is the computer connected via digital output to your receiver/prepro? If you're using analog outputs, you've still got the Windows volume control and possibly another gain stage or two between REW and the decoder. This could definitely explain why you're just barely making it with the wattage listed. There are probably other sources of test tracks, but Soho54 created a downloadable DVD ISO many moons ago that contained individual DD 5.1 channels of white/pink noise, including LFE along with individual sine waves down to 10Hz IIRC. The link is probably still in the forum archives, but I don't know if the ISO is still being hosted at the link.