Thanks to SVXdc for the heads up to this
NASIOC thread which highlights a couple of Impreza users who found secret menus on their stock head units.
After a little experimentation I found that it also works on Legacy head units! Specifically this process:
"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"
My head unit model number is CE617U1 which is the base and up for 2013 and the premium and up for 2012 (non-hk/non-nav). So those of you who want to keep your stock units and improve the sound, especially if you have an amp/upgraded speakers, then fear factory EQ no more!
Also feel free to experiment with other head units to see if similar options are available in other models. Don't forget to post here if you find anything!