I just installed this sub under the driver seat. Its tight but it fits with minor modification. I also put a PPI amp under the passenger seat. It runs the front Kappa's. Sounds good. Just loud enough. My goal was to get better sound without adding too much weight. With the help of dynomat in the doors, I'll call this a success.

 

I was actually looking at this one today on Crutchfield. I assume you arent running it off the stock head unit though???