This 600W Kicker sub and the 400W rms Alpine mono amp sound better than the Monsoon in my wife's car and are fairly easy and inexpensive to install! The square design makes this subwoofer have the same amount of surface space as a "12" round sub. Cost was in the ballpark of the Monsoon system, around $400. (Best Buy and eBay) The speaker grill has to be purchased seperately unfortunately. The speaker has Dual 2 ohm voice coils.
The plastic interior panel behind the seats has to be removed as well as the radio to perform the installation, but that's not too difficult.
The mounting base is made of 3/4" plywood covered with batting and "sand" colored or black leather, or faux leather. A square hole is cut out behind the passenger seat where the arched area is. The base can be reinforced inside the speaker cutout for additional support. The resulting enclosure needs to be insulated to keep the sound waves from bouncing around inside the trunk area. The base keeps the speaker from bottoming out when mounted.
The Alpine amp fits behind the drivers side in the existing amp mounting area. It's a great amp with smaller than average dimensions.
The amp needs to be directly wired to the car battery and the power on switching for the amp can be wired into the Head unit harness. The wiring harness pin out can be found on the forum. I just set it and forget it on the amp output settings since the amp is completely hidden behind the driver's seat inside the panel. I tapped into the rear speakers for the amp's audio input.
This system has worked well for over a year and provides a lot of bang for the buck. The door speakers could be upgraded while you are at it. One needn't pay a fortune for a fiberglass box or waste more than five or six hundred bucks for great sound in their Sol.