Low-Buck Base setup

[Tynee]

Good morning truck audiophiles. I just replaced the head unit in my '08 Crew Cab with a Kenwood DDX372BT:

Kenwood Head Unit

I replaced the front door speakers and tweeters with Alpine SPS-610C's:

Front Doors and Tweeters

And the rear door speakers with Polk DB651S's:

Rear Doors


I'm powering the speakers directly from the head unit. I'd like to fill out the low end that I'm currently missing, but don't want to spend a ton. When I took this question to the local car audio shop, their recommendation was a $650 setup. I'm looking to get by for less than $200.

I'm thinking about this setup under the rear seat:

P300-10
Can anybody point me in the right direction for a separate 10" Sub, amp, and enclosure that would beat the sound quality of this Rockford Fosgate setup without being TOO much more expensive?