Yes, it sounds like because your room is so big, you had to turn the gain knob on the back to 3/4. That, combined with opening it up to 150 Hz, caused compression.

I've never seen compression curves for the 500 but here they are for an older version of the 800 across its entire usable band.

http://www.hometheatershack.com/forums/sub-zone-subwoofer-testing/62085-axiom-ep800.html

House of the Rising Sone
Out in the mid or far field
Linearity and mid-woofers are over-rated