Powered subwoofers are comprised of a subwoofer that produces extreme low frequencies (you know, the sound that punches you in the gut), as well as a built-in power amplifier, so the unit can operate on its own without the need for an external amplifier. We always suggest in the strongest terms that you get an electronic crossover so that you may exercise complete control over your whole pro audio loudspeaker system.
Powered subwoofers come in a wide variety of sizes with woofers as small as 8 inches or less, to behemoths comprised of 21 or 24 inch woofers. At the end of the day, you make you selection based on what size crowd and what type of environment will you play in 80% of the time. If you run into a gig that needs more ooomph than your pro audio system can handle, you can always rent a few extra speakers.
We have several powered subwoofers that we have come to depend on over the years, and we can recommend them without hesitation. The Yorkville LS801P, the Yorkville UCS1P, the JBL PRX718XLF, the QSC KW181 and the RCF SUB718-AS.