We can see the detailed measurements of the FV15HP on data-bass. It appears that the THD stayed well under control (basically under 10% from 30 to 35 Hz and up depending on one port or two port mode), even while it is pushed close to its output limit. We also see that the dominant component of the THD is the second harmonic from 80 Hz until about 28 Hz in both modes. Going lower in frequency, the 3rd order harmonic becomes the dominant component and rises rapidly. This is when the sub is pushed close to its output limits. It seems that this was likely by design, because when we get below 30 Hz we are less hearing the sound and more feeling it and the 3rd order harmonic in that range shouldn't stand out to us like it would at higher frequencies. People find the 2nd order harmonic distortion, less objectionable and some even find it pleasing. Was the F25 designed with a similar approach to the distortion curves, keeping THD under 10% until close to the output limits and the second order distortion being the dominant component from 28 to 30 Hz and up, except distortion being larger than the FV15HP from 40 Hz and below when the F25 is pushed close to its limits (due to the fact that it is a sealed design)?