Well, first you have to decide how much bass. Booming or balanced with the stock system. If you just want a little more bass, some people have tried
Kenwood's WooX . The nice thing about this is that it should fit under the driver's seat and it has a built-in amp. If you want more bass, but still hidden, you can go with
JL Audio's Stealthbox . This would require an amp to power it and the
JL Audio 250/1 provides the perfect amount of power
. If you go the stealthbox route, I recommend finding a reputable car audio store in your area because JL does not honor warranties on internet sales. As always, you can come back here and discuss any prices if you are skeptical. I have the stealthbox and love it, but if you never plan to upgrade the other speakers it may be more bass than you need. Hope this helps