All distortion measurements were made within an 80 kHz bandwidth.

At 20 Hz and 100 dB output, THD+N was a little less than 6%, which is terrific performance.

At 40 Hz, distortion dropped to about 2%. Again, this is very good performance.

Plotting THD+N vs. Frequency, we can see that distortion stayed well below 5% from about 12 Hz to 130 Hz. This is actually quite amazing for having the driver in a relatively small enclosure. The peak at 140 Hz is an anomaly that I can't explain. But, it is well below any crossover frequency that you are likely to use, so it is irrelevant.

For frequency response, I was limited to measuring from 3 meters away due to the fact that sound was coming from opposite sides of the subwoofer at all times. I tried to smooth the response using the three EQ bands, but was only able to make minor improvements. You can see from this how much effect a room has on a speaker's response, and my room has a lot of wall treatments. Taking into account the peaks and valleys that are obviously room effects, I would say the MKVI is flat down to 20 Hz.

I was able to obtain a maximum output (measured 1 foot from the passive radiator) of 110 dB at 15 Hz, and 115 dB at 20 Hz.