Lethargic Man (anag.)

I did a bit of digging, and it does look like the Music 500 has a powerfully weird DAC design. I saw a spec sheet online that said it sampled at about 700KHz, which worked out at about 47KHz per channel. This is best described as bloody weird -- I suspect they have avoided the need for digitally summing the signal by basically multiplexing the channels into a faster DAC, then assuming that the frequency response of the output would effectively do the summing for them. Yes, this would work, but OMG, the unwanted high frequency harmonics at unrelated frequencies that this would kick out is really not funny. I think you may need to go the heavy weapons route for this one, and use a bit of active electronics. What you need is an analogue 'brick wall' filter, which is really aggressive outside audible frequencies. Happily, you can get chips that do this so you don't have to build a really complicated multi-stage filter from opamps these days. I will upload a circuit diagram for you shortly -- I used the Linear Technology FilterCAD software to design an elliptical filter that cuts of at 18KHz (better than FM radio, which knees at 15KHz). It falls off extremely rapidly down to at least 70db of cut, which should be enough to drop any aliasing below the noise floor. I'll post screen grabs with the circuit diagram and frequency plot shortly.