While a complete sound system was out of the budget, I did decide to add a sub to solve the bass "problem".
I went with the large sub kit from SubThump (SubThump 12” sub kit - http://subthump.com/product_info.php?products_id=449
). It sounds great. I still have some adjusting to do, but I haven't driven the car much since this was installed.
A few notes on the kit and installation: MOST of the hardware needed for installation is included. I state most as I was under the assumption (yes, we all know what happens when you ass-u-me) everything needed was included. Just a small gripe, but you will need screws to attach the amp to the box, and screws to attach the sub to the box. Being a kit, I would have expected these to be included. Also of note is the wiring kit. While it does include everything needed, the RCA cable is medicore quality at best. The RCA plugs don't fit very tightly. I end up taping them to ensure they don't vibrate off.
Also, per the instructions, you need to slide the front of the box towards the center of the trunk. After several attempts (and breaks to walk away and gather myself), I could not get the rear of the box to slide past the stud (cargo net threaded stud). I finally pushed it straight in as far as it would go, then pushed the front in first...at which point the rear slide right into place. A few love taps with a rubber mallet and installation was complete.
And I forgot to mention that while the interior was apart, we ran the cable for the remote bass knob. It's mounted in the center console by the cup holders (as I've seen in someone elses pictures).