Not that there are many passive subwoofers these days, but I'd thought I'd share how mine is powered. Instead of connecting the main L/R speaker outputs of my receiver to the inputs of the subwoofer, and the speakers to the subwoofer, I have my Yamaha receiver's subwoofer output connected to a 25+-year-old Harmon Kardon amplifier (bought it from a friend a few years ago for $15). The combination produces some nice foundation-shaking bass.

Passive subwoofers were popular when I got mine about 13 years ago. Nowdays, most Dolby Digital/DTS receivers have a line-level subwoofer output, which requires either a subwoofer with the amp built in (very common now) or a separate amplifier. Note that if I connected the subwoofer to the L/R speaker outlets of my receiver, I wouldn't have a true ".1" for my 6.1. 

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