sontakke, I'm not sure about the 540's ... I have the 640s, so I can't say 100% for sure. Since you don't have the SPDIF cable and back panel, I would recommend forgetting about that approach. You say the rear out only has a Left and Right ??? Do you mean a Line In and Line Out, plus Microphone ? The 640s are purely analog and I simply plug the green jack into the green Line Out jack. A digital connection is not an option you should require with the hardware you have. If you have a special need for digital or the SPDIF then we can work out those details.