OOMSAP, another story

Another pair of Quad was on an auction site, now sitting in the middle of old ones. That is listed as ESL-63Pro Junk, because one of them makes bussing sound, which makes bussing sound as explained.
One ESL-63Pro is now located at the middle of ESL-57 pair through channel mixer and 6dB/Oct LPF at 200Hz. AWCS only works below 50Hz (12dB/Oct @ 50Hz). I tried to adjust each level and crossover frequency without any test equipment, and that was so far successful.

I borrowed measurement microphone with a microphone preamplifier and 24bit/192KHz ADC.
The measurement result is unusual. The response is +3dB@100Hz, +5db@63Hz, and +5dB at 16Hz. That is practically flat. However, it is flat up to 16Hz. Connecting the cannon shaped blower makes obvious difference even though I barely hear its sound by itself, except rattle sound from some furniture at very high volume (but yet barely heard). The OOMSAP does not know the reason why the air vibration far below the audible frequency because OOMSAP is stupid.