I am fairly certain that armadyl should out-DPS void in the fight caves at 75 range. I'll do some calculations and edit this post in a bit with what I find.
EDIT: Well, all of the DPS calculations were extremely convoluted and I'm not even sure if I did them right, but it looks like void is superior to armadyl strictly dps-wise against all of the fight caves except Jad, where it is a little bit worse. But depending on other factors like your experience with doing the caves, your defence/prayer level, etc. you might find it more worthwhile to use different gear like karils/arma and veracs.