Figure out what you want to use a bass amp for first. Remember that, the lower the frequency, the larger the sound waves are. If you get yourself a nice, powerful amp and cabs with the really large (like 18") speakers in it, you're going to be able to shake the house but it probably won't sound good to YOU. Why? 'Cause you're too close. If you're playing live, then maybe this will make the audience happy (although most of the time you'll be put through a DI straight to the FOH board).
My advice is to forget 12" speakers - let the guitarists use those. Instead, get a cab with two 10"-ers in it (plus a tweeter) and, if needed, add a second cab with a couple of 15's - best of both worlds. Here was my setup (with a Euphonics Audio iAmp-800, capable of 1,000 watts at 2ohms which was how I ran it). The cabs are exactly as mentioned above and come from Avatar Speakers - a highly recommended company for quality gear, nice prices and excellent customer service.
