Let me explain this. Usually these cheap 5.1 systems have six amplifiers inside the subwoofer enclosure - one for the subwoofer and the rest for satellites and center channel. You usually connect the 5.1 analog out of dvd player to the 5.1 inputs at the back of the subwoofer. But this is not the case with serious home theater systems. Their subwoofers (like the PSW110) have a single amplifier (and so known as powered subwoofer) only. For the 5 channels you need a separate amplifier. Here comes the role of AVR which is the center of the HT system. It can accept multiple inputs in the form of HDMI, optical, coaxial and 2 channel analog. The AVR then decodes the audio signal received from the hdmi or optical/coaxial output of your dvd or blu-ray player, amplifies and outputs through 5 or 7 amplifiers depending on 5.1 or 7.1 system. The AVR has no amplifier for subwoofer. Instead it provides a subwoofer line out which you connect to line-in of the powered subwoofer. This means a 5.1 AVR has only 5 amplifiers. However some entry level HTs like Onkyo 3400/3500 has all the 5.1 amps inside the avr unit and the subwoofer is passive (no amplifier).