>>12148842
I searched for any change in the exp penalty for levels and couldn't find anything outside of >lvl70 penalties
https://www.theamazonbasin.com/wiki/index.php?title=Patch
Doom knights are like level 59 on nm, so you'd only be getting 5% exp from them.
Dark ones (act one fallen) in the nightmare catacombs should be around level 35, which is right at the top end.
Do some nightmare tristram runs.