Summary: This is an older model of Zvox's 555 current model. The main difference is the inputs. The 550 has only an RCA connection so if your set up uses an optical output, you will need an adapter. They cost about $30. No big deal. The unit works great! It is not (as Zvox states) a substitute for a 5.1 setup but has enough control to really open up the sound stage and give a great 2.1 sound.