Low RPM
DK
I am not shure I do understand, but the theory (and my math) show that AOA increases up to the optimal point. Increase is necessary because overall the rotor is not very charged at reasonable cruise speed, and thus below optimal loading. One exception is the retreating tip. Reducing RPM will require you to pull collective to maintain speed, this should convince you that AOA increases.
Added : drag is lower because average speed of blades is reduced by lower RPM. A low loaded profile has near constant drag coefficient, so only dependent on speed and not really on AOA.
d3