I got a surround reciever off ebay for 63 bucks, add an old c/d player and golden, as you already have the speakers.. hell a rca plug to a ipod or mp3 player cable is 10 bucks..
any inverter will add noise(hum) to the sound as they are not made for the power needed,(at least the cheap ones)
my 75 watt per channel surround sound (5.1 channel) was 63 bucks shipped, my ipod cable was 10. my c/d player was free I have a few extras now that dvd/bluerays play c/d's
and old car speakers..
and a senhizer(sp?) wireless headphones, for the burning the midnight oil ,
ebays your friend here.. my radioshack reciever will be plenty loud.. and last forever and for 63 bucks.. deal..