i've heard good things about the VIBE underseat sub.
Search Gazza190 posts as he's just fitted one in the rear of his tourer and I plan on doing the same with the passive version (his is active).
Also heard the Alpine is pretty good for the size as well.
They'll never beat a 10, 12 or 15 sub in a large enclosure, but they are a hassle taking up all the space.
I'm going to fit a vibe for day to day use (it's an 8" jobby), but leave it with a banana plug port so I can put in my 12" ADS sub for when I know I want loud sound (like a show or long trip etc). Hopefully best of both worlds then
