...Driver issue? The 6000 series is perfectly capable of sending 8 channel audio via HDMI. The fact the speaker test isn't working right kinda points to drivers.
As a quick test, do a 7.1 sound test, and check to see if the side speakers are getting mapped to the rears, and vica versa. On PC audio spec, the side and rear speakers are the reverse of what they are in Dolby/DTS spec, so its possible your amp is getting confused and mapping uncompressed audio signals incorrectly.
only thing I can think of is reciever had a funny 5 minutes & worked when it wasnt supposed to, all other settings have remained the same, but had this runnig a few hours now & touch wood, all is fine, figers crossed it remains that way ?