thx gave their approval to the bgw m2200 self-powered subwoofer introduced in 1993. the subwoofer contained four 15" drivers, crossover circuitry, and an internal amplifier based on the grand touring series design. thx quality assurance also approved the bgw m1100 subwoofer (basically half of an m2200) in 1995 and the millennium amplifier line in 1996.