Things to keep in mind if you are using Meridian comms:

If the HD621 is on, the G68 is on.

If the HD621 switches inputs, the G68 switches inputs.

You would need to turn off the amplifiers manually in order to have the sound only come out of the TV.

If you want to listen to surround content you would have to turn the TV down or mute it manually.

If you are using DSP speakers, they will also respond to power, input , and volume status.

Unless you are using a control system that can control the HD621, G68, and the DSP speakers discretely, this won't be much fun.

The simple answer is just use surround sound.

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