It is on the Sony site. I can’t link you to it directly because the manual is a PDF.
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We really have created a nightmare. That is 80 pages of instructions for sound bar!
That unit does not have a center channel. Interestingly the crossovers are active.
Now what must be happening is a failure to mix down.
I would see if the problem also exists on the optical connection as well as HDMI.
Try a reset, page 68 I think.
I had the impression this problem just involves Direct TV. If this is so, just go the the Direct TV menu and instruct it to mix multi channel and just output two channel.
You could do that with Direct TV boxes I have owned. If any other devices connected can be set to mix multichannel to 2 channel stereo I would do that as well.
This is basically a two channel device, but you can connect rear speakers. I would suspect making sure it only receives stereo signals will solve your problem.