[brucek]

I set my volume at reference (75db) and measured the level output for each frequency test tone (10Hz, 12Hz, 14Hz, etc.)

Your readings are all around 100dB though, not 75dB.

I measured with signal send to Sub only (red) and with signal send to both (green).

And adding your mains increased the level at 10Hz by over 20dB. Does that seem right to you?

brucek