Certified to THX ULTRA2 and THX SELECT2 standards, the formidable Ci200QSb-THX in-wall subwoofer delivers deep bass and theatre thumping output capabilities, matched to the uncanny ability to deliver all the drama and emotional intensity of full theatre sound quality. With a powerful high-output 200mm (8in.) sub-bass driver, the Ci200QSb-THX can be used in passive mode to supplement the LF output of KEF’s Ci series in-wall or in-ceiling speakers. In Active mode, when 2 pair of the Ci200QSb-THX subwoofer are powered by the powerful 250Wx 2 KEF KASA500 System Amplifier the performance meets THX Select2 specification. When 4 pair of the Ci200QSb-THX are powered by 2 KEF KASA500 System Amplifiers the performance meets THX Ultra2 specification.