On the Bench 

At 50 Hz and 100 dB output at 1 foot distance, THD+N was less than 5%. This is pretty good for such a small sub. Remember, the MicroVee is less than 10" on any dimension. The second harmonic is larger than the third, which means there is still some output capability left. We are not at the limits here. All measurements were made using just one MicroVee (I had two on hand for some of the listening tests).

Setting 50 Hz at 100 dB, I measured THD+N vs. Frequency. The MicroVee is designed to roll off below about 28 Hz, and you can see that THD goes up below this frequency. There is a peak in distortion at about 40 Hz.

The room response indicates that output drops off below 50 Hz. Use of a subwoofer EQ, such as those found in many receivers these days, could bring the output at 50 Hz and above down to match the output between 30 Hz and 50 Hz, which would flatten the overall deep response.