I agree that Logitech is diluting and exploiting THX as a brand. If the multimedia certification for their desktop systems were sincere, that is a flat response from 35 Hz to 20 kHz with 85 dB continuous and 105 dB peaks with a flat response, low noise, and low distortion, well that isn't that bad for a desktop system. The problem is I don't know if I can trust that is genuine. I don't know how much pliability there is within those guide posts, ie how much distortion is permitted? How loose can the frequency response be within say, a +/- 2 dB window? and so on.
Also, I don't see anything that says the f3 is 35 Hz, only that the system can play down to 35 Hz. It could be a f6, or could just be the fs. Who knows. And that is part of the problem.