I am connecting logitech z906 to my sony led tv (Sony KDL-40NX650) through optical cable ,but not getting satified 5.1 surround sound ( rear speakers make low noise),whereas it works perfect on 4.1 effect , my tv has stereo ( 2.1) digital output, so what do i need between tv and logitech z906 to get maximum output from my rear speaker during 5.1 setup. Should I go for decoder which will convert 2.1 stereo output into 5.1 for logitech Z906. Or is there any alternative