Here is a graph of Audyssey DEQ on (top purple) vs off (bottom green):
The slight treble boost is above 10kHz, which is usually not apparent or audible to me.
The major boost is in the bass is from 20Hz-150Hz.
For movies I listen to 80-100dB volume. So DEQ doesn't just boost bass at low volumes for me. It boosts the bass even at full volume for me.