Try connecting the HD box to the HDMI on the HTS
I am going off of how the my A550 works, but I am sure yours is the same. The A550 will only output 5.1 sound from the optical output when the TV's internal tuner is being used. If any sound is being sent through the HDMI cables, the TV will down convert the audio to 2 channel sound.
Option 1)Looking at the HT-Z512 online, it has a HDMI pass-through on it. You could connect the HDMI from the Dish box to the HTS, then, connect the HTS to the TV. This should work, and would save a optical cable. But, some devices do not like any kind of switch between it and the TV. So if it fails to work, you will have to go with my second option.
Option 2)Connect the Dish box directly to the TV with an HDMI cable. Then use an optical cable to connect the Dish box to the receiver. This is the way I would actually connect up your system. Connected this way, you will get 5.1 surround from your Dish box, and you will leave the HDMI port open on your receiver if (when) you decide to get a Blu-ray. When you do, I would connect the Blu-ray to the receiver using the HDMI in on the receiver. That way, you can here the HD audio formats available on Blu-ray without having to use 6 or 8 RCA cables.
Also, make sure you set the audio digital out format on your Dish box. I do not have the box you have, but, when I had Dish, my box had an option for "PCM" or "Dolby" digital out. If you have your Dish box set for PCM, you will only get 2 channel audio.
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