Unpowered subwoofers are better than powered ones. There are several reasons for this:
First of all, you don't have the amplifier being so vibrated as one that is built into the cabinet, so the amplifier is less likely to fail. Second, if the amp ever does fail, it is much easier to replace if it is not built in. Third, you can select whatever power you want with it. If you have an old amplifier (in a receiver or whatever) that can handle the impedance of the subwoofer, you don't have to buy a new amplifier at all. With, for example, the unpowered SVS subwoofers, they are fairly efficient, and you may be surprised by how much bass you can get from a low powered amp. Remember, it takes a doubling of power to get just a 3 dB increase in volume, so if you had half the power of the installed amp, you would only lose 3 dB in possible volume (forgetting about peaks for a moment, which may mean you are less likely to miss anything, as you are unlikely to be constantly driving the woofer at maximum power), and you lose only 6 dB of maximum volume if you have only 1/4 of the power of the built in amp, and you lose only 9 dB of maximum volume if you have only 1/8 of the power of the built in amp. So even if your extra amplifier is not very high powered, you can still get good results. And, of course, you can select an amplifier that is as powerful as the built in one, or even more powerful, if you wanted.
Also, if you are using the subwoofer with a Dolby Digital surround system, your receiver/preamp/processor has a dedicated subwoofer output that deals with the crossover for you, so paying for a crossover built into the subwoofer is a waste of your money. So all you need is a power amp with a passive sub.
Fourth, many people experience difficulties with "automatic on" subwoofers. Those stupid auto signal sensing things are not a good idea, as it must turn on only AFTER the music starts. And then they shut off if the music is quiet for an extended period (or with a subwoofer only signal, if there is no deep bass for a while).
The solution to the problem is to buy unpowered subwoofers, use a separate power amp, and an automatic switched outlet strip, such as:
http://www.smarthomeusa.com/Shop/Sma...pecifications/
where one plugs in one's receiver or preamp into the "control outlet", and the subwoofer power amp into one of the automatically switching outlets. It works great. I know, I am using one of these strips with the power amp for my SVS CS-Ultra subwoofers.
With the setup I suggest, it turns on as soon as the receiver or preamp turns on, and turns off only when the receiver or preamp is shut off. So it does exactly what it should do, unlike those stupid signal sensing automatic on circuits.
I do not plan on ever getting another subwoofer with a built in amp. In my opinion, it is just asking for trouble.