Well, I've spent some time on the 708 manual. In particular p.38 describes the listening modes and appears to clearly indicate that Direct applies to multi-channel, not just 2-channel sources. If the back driver output from the DSP is plugged into the correct multi-channel surround inputs on the receiver, it should be amplified and come out the surround terminals. There is also a separate "Multich" mode, but the description indicates this is for PCM(digital)use, although it could be tried.

It should be verified that the back drivers are operating by connecting the front speaker wire to them. Also, the back driver speaker wire should be connected to the front drivers to confirm that there's no sound.

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Enjoy the music, not the equipment.