I think it's the factory HU. As has been noted, some of those systems actually restrict the bass output as it goes up. I would look into an LOC that has acubass. Call Audiocontrol, ask them if the LC1pro would do the trick or not. You might want to conisder upgrading the HU with someothing better like this used on
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Or this for some very good sound refurb for $249 (about $50-200 off new)
Compact rear chassis for easy installation. Resistive touchscreen with anti-glare. SiriusXM ready.
Unless you need disc playback then you in the $400 plus.
WHile I think the LC2i might help, I'd be looking to replace the HU. At the end of the day, you still got lipstick on a pig, upgrade the pig,