From the NHT link below:
SW2P - 1992
First power subwoofer package
120 Watt discrete amplifier
10” Long throw Polypropylene woofer
High output vented enclosure
In 1992, NHT introduces the SW2Pi. It is the worlds first powered home audio subwoofer. It consisted of a matched SA-2 discrete mono power amplifier and an SW2Si 10" woofer in a high output vented enclosure. The SW2Pi subwoofer system was capable of accurate low frequency reproduction to 27Hz, with high output and low distortion.
http://www.nhthifi.com/history.html
Looks like my boys at NHT may be fibbing. M&K does say 1977. NHT uses such words as "package", and "home audio."
From the M&K site:
In 1977 M&K debuts the World’s very first “Powered” Subwoofer! This model was called the “Volkswoofer.” The validity of this concept is evident today as virtually all subwoofers by all manufacturers are self-powered