Why passive speakers?The AP4 passive speakers are full-sounding bookshelf speakers in a satellite-sized package. The goal with AP4 was to design a small but powerful bookshelf speaker for people that already have surround receivers or amplifiers and are looking for the same Audioengine sound and quality in a passive loudspeaker.Already have an amplifier?"If you already have an power amp or don t want to give up that big surround receiver then the AP4 is for you. The AP4 has excellent bass response for its size, which is evident when used either as stereo speakers or as satellite surround speakers in your home theater setup.