'11 Yaris - any way to disable bass reduction at high volumes?

[adrew]

The head unit in my '11 Yaris starts reducing the bass after the volume is raised past level 40 or so.

This isn't really noticeable with CDs or the radio since I don't really put it louder than 40 or so with those sources. But sometimes I stream music with my phone -- and it tends to put out a lower signal level, even at maximum volume, so I have to crank the head unit all the way to the max to get decent volume.

I assume the head unit is doing this to reduce distortion from the factory speakers, but when listening to music from my phone it just ends up sounding flat, with no oomph, like a clock radio or something.

Is there a magic key combination to disable the bass management?