I'm definitely not grasping something here, I'm just not sure what.
I thought that if I was going to use the Sony receiver and send the unamplified signal out to a power amp, that the Sony was the decoder you mention. So for television purposes, the Sony would act as the DAC, and instead of using its internal amplification, use a dedicated external amp.
I do have the Sony XA-5400ES, which would go to the receiver via HDMI, which is definitely the optimal connection between the two. I'm assuming that sending this signal out to external amps after its reached the receiver would not be at a significant SQ loss (after all the whole point would be better SQ) ?