I build a subwoofer system called The Swarm that consists of four small passive subs and a separate amplifier that drives all four. On occasion I have sold the subs without the amp, but I don't sell the subs in ones and twos - only four at a time.
Imo there are two significant advantages to not having amps inside the subs. First, in this day and age of lead-free solder, electronic components like subwoofer amps are significantly more susceptible to failure if they are subjected to vibrations. Second, if the amp does fail, it's much easier to repair or replace an outboard amp than to repair or replace an amp that's built into a sub.
Parts Express sells subwoofer boxes that you can install a driver in and end up with a pretty darn good sealed-box passive subwoofer.
Duke