I have been installing systems for quite some time now and I just have to tell everyone about how great Alpines PLV-7 is.
I was running an Alpine Type R 12 with two Alpine MRD-M500 amps for a total of about 1600 watts. It sounded awesome but it also took up a lot of room. As I am a fan of Alpine's products I regularly view their Alpine-USA website. I came accross their LAT PLV-7. A new technology in powered subwoofers. It uses 6 flat 7" woofers powered by a built in 500 watt RMS PDX amp to produce what they claim will equal 2 12" woofers. I gave in and checked it out in a local store (after finally found one that stocks it). I was definately impressed. It's designed for SUV's but in a ten by ten room, IT ROCKED. I still wasn't sure about the $799 price tag. So I did some checking on Ebay and found one brand new for $550 shipped. You can also find them pretty cheap if you go to the suppliers. It's harder to locate them than it is a store but it saved me $300.
If you are getting to the point that I was, where you still want great sound but are tired of the huge box and amp racks, look into this. I have it in the trunk of a Civic coupe and it sounds great. Best of all I still have a ton of trunk space.
Other great features about the PLV is its partially rubber wrapped box. As well as the plexiglass front that lets you see the trippy interior set-up. It comes with mounting plates that if you install them properly make it pretty hard for thieves to steal. It also keeps it from sliding all over the place under hard driving. On top of all that it comes with a remote controlled gain adjuster. If it's too loud (or more likely not loud enough) just give it a twist and let the smooth bass fill your ears.
For my next big project I plan on purchasing several of them to do a revolutionary design in a custom competition system. With the amount of bass these things produce with such a small package, the applications are endless.