But if luck affects both quantity and quality, wouldn't killing an Ursix nullify the quantity aspect of luck, but not the quality aspect?
In my experience, an Ursix always drops 2 items. I'm certainly not expecting to get 3 items to drop when running with 190% luck, but shouldn't repeatedly killing an Ursix with 190% luck result in a greater proportion of rare items as compared to repeatedly killing an Ursix with 0% luck?
You're technically correct (which is the best kind really!)... if you could isolate just those things your luck will factor into the final rarity that you get.
However, there are a whole bunch of other factors... like if the Ursix is the mini-boss for an encounter it might have been scripted to have slightly better rewards.
Basically, everything being equal it should help you get slightly better stuff - but it's complicated because stuff is rarely the same situationally. 😊