That was what I was saying "it is too good to be true" (without the aid from sub). I believe you know quite well that, in order to boost 6dB, you need to double the volume. So I wonder why do you even make fun 😀 of people when they question that as below. It is simply not doable either practically or in theory. That also lead to my next question in your post where you mentioned: "... Take my modest speakers. They use 6.5" woofers and a ported cabinet. Would you guess they went down to 20 Hz? They do..." How you achieve a flat response down to 20 Hz there? What is the frequency rating for your modest speakers (Squires SNR-1?)?
there’s no way speakers that are rated at 41 HZ are going to go down to 20 HZ in your room what kind of measurements are you using?
The words of those who don’t have a lot of practical😀 experience.