YES, WE CAN!
I just picked up a 2013 Crosstrek with the CM621UB HU. (Do I get partial credit for having most of an Impreza?) By piecing together info from various posts here, the service manual, and documents filed with the FCC, I figured out that it *is* possible to disable both the loudness and fixed EQ.
To turn loudness on or off:
With the unit ON, press and hold 1, 6, and TUNE (right knob.) The display shows LOUD OFF or LOUD ON.
To disable fixed EQ:
With the unit OFF, press one of these combinations of buttons:
1 + 2 + TUNE for SEDAN
1 + 3 + TUNE for WAGON
1 + 4 + TUNE for THROUGH MODE
With these "features" disabled, it sounds significantly less awful, especially at lower volume levels. This should make it a much better candidate for use with upgraded speakers, or a line out converter and amps. From the schematics, I think it would be fairly easy to add proper RCA line outputs as well.
I don't know if these keys will work on earlier models, but hopefully they do.
Fantastic forum you've got here!