hi Chris & Skip.
i was testing out the blackman 7 bandwidth filter i installed which is set to 20Khz Bandwidth (10Khz each side band) at a high order of magnitude.
the blackman filter provides a sharp drop off at 10Khz versus ST's built in filter which has a rolling dB drop off staring from 9Khz to 10Khz, i prefer the blackman filter as its flat across the board.
though i wasnt too thrilled about the minor aliasing at the upper audio frequency spectrum during a sweep test (with and without the blackman filter), it wont be detrimental to music and speech, its a minor issue at this point.
during the test, i sent 20Hz and 40Hz sine wave tones and observed the results at SDR demod and was very satisfied with the ending results.
you certainly would have seen your sub-woofer moving at 20Hz provided your receiver is capable of receiving the low end of the audio spectrum