Maybe I can get hold of a box that will output 8 channel PCM from HDMI as 4 stereo pairs of SPDIF. If you are interested, please let me know.

@George: Please correct me if I am wrong, but as long as I have one DSP speaker configured as master (currently my DSP5000c), and the other speakers connected through a S-Box (currently on the back of my 565, but there is a separate box available from M), I can control the volume of all DSP speakers simultaneously.