Turn the LOUD feature "ON" on the headunit. it made a huge difference on mine. even though it says its only for low volume.
try PUT the SLA up so u have more volume, if its low...
i had a similar problem with a similar setup (deh-p4050). i played around with the settings and managed to get nice loud bass.
Also as said above, face the sub towards the back of the car. i tried all angles and found that it sounded best that way.