Originally Posted by sobejoe
I have a 8in kicker ... and [an Alpine MRP-F300] amp ...
The problem is bass output. Songs that should have a lot of bass dont.
Which speakers did you tap for the sub amp?
Recent Subaru OEM HUs (including the one in your 2011 WRX) attenuate bass frequencies sent to the rear channels as you raise the volume, making the fronts a better choice for a subwoofer.
However, if you use the hidden menus to disable the default EQ, you can use the rear channels for a sub. See this thread:
Hidden menu for 2011+ "premium" head unit?