This seems to be a compatibility issue, The NUC basically is not detecting all features through the HDMI, You have noted that it works ok directly to the monitor.
You can try with different cables since cable quality is an important factor, also you would need to ensure that this receiver is HDCP compliant as well..
I've changed the HDMI cable, without any effect.
I try openelec, and miracle :
- i can change the aspect ratio
- AC3 and DTS (including HD) are ok.
Conclusion : Intel's video drivers are a big failure.