Hi,
feedback loop? No no .. certainly not.
I have the original DMM cable in operation. So that is not the issue. Also, when AC3 downmix is off, then no sound can be heard on the TV's Toslink-out. When downmix is on, then I have DD 2.0, meaning Dolby Surround (mono in the rear). This configuration certainly make sense, because besides the DM800, I my TV has a built-in DVB-S receiver. Thru this, I have a twin-view and PIP, too. With the AC3 Amplifier connected to my DM800, I can't use the TV DVB-S receiver.
Mamba