20Hz: 114dB at 4.5% THD. I did not push any further, as the panes in my windows were moving about 1/16" at the time. I think they would have broken if I went further. Above 20Hz, you can get damned near as much as you want dB-wise, all clean. Basically, from 16 or so Hz on up, you have strong bass, from 20Hz on up, INSANE bass. I designed the system to give me great sound at -10dB from reference (with Dynamic EQ frequency correction to account for equal loudness curves), which it does with flying colors flat down to around 16Hz before dropping off. Useful output down to 12Hz. All at less than 5% THD. In a sweep and distortion test of a $9950 sub, the Genelec HTS6... the Genelec runs over 10% THD at 115dB. This is significant, in that a sub almost 20 times less expensive to build can seriously compare with one of the best subs available.