Not sure, the only thing is you may be running the sub to hot on the volume (gain) control. On my Denon 2805 with EP600, to calibrate at 75dB with my SPL meter the knob on the sub is at about 6-7 o'clock, and the receiver is at around -6dB's. If I put my Denon on 0db and the volume gain on the sub at 9 o'clock like you, it would be way to high and the woofer would be going nuts.

I think my receiver would have to be on about -12dB with 9 o'clock sub setting to get 75dB on the meter.

M80s VP180 4xM22ow 4xM3ic EP600 2xEP350
AnthemAVM60 Outlaw7700 EmoA500 Epson5040UB FluanceRT85