Channel 7/8 on the DM-810 has multiple sources it could get it’s signal from. Depending on what you have connected, and the status of those inputs will dictate which input the DM-810 will pay attention to.
Specifically, how the analog 7/8 input works with both digital inputs (optical and coax) and the option port (Bluetooth).
If you have analog connections on 7/8, they will be immediately taken over if it senses a signal on the digital input(coax or digital).
If the 7/8 input has an analog signal, and then senses a signal on the option port(Bluetooth) it will then mix both the bluetooth signal and the analog signal together. In this situation, you would then have to turn off, or stop sending the signal on the 7/8 input.