Thanks. Mine is PowerColor ATI5450. I thought about the same before, but end up the driver that I download from PowerColor is the same ATI driver.
BUT, I've just get it to work 10 mins. ago. The solution is similar to the the master of this post. I updated the HDMI driver to RealTek. AND ALSO all RealTek audio drivers which my motherboard used.
Now 1 problem fixed (which is great), another comes. Now Power DVD10 has a new audio option, "decoded by external device", which allows DTS-HD code passthrough and the LAMP lights up! when it playback a disk file. But unfortunately this option is not available when its idle. So, which means I need to switch it on everytime I play a file. Is that normal? Also, when I play the files via Power DVD8, this option is not even available. It has only got "DTS/AC3 passthrough", which means if I play the DTS-HD format, the amp only get the DTS.
You should try to download the ATI hdmi driver to replace the Realtek. Mine is also same to you, everything goes OK passthough amp 607.
Or you can use TMT5 as well.
Hmm... I tried to uninstall the RealTek HDMI driver for ATI, but the same old problem came back and the 3rd options from Power DVD 10 was not available immediately. So, I put the RealTek HDMI driver back to it and those options comes back to me.... so, I don't know. Now, as long as it works, I'm happy. And the HD sound is great!