The Rega Apheta 3 has a .35mv output
The setting on your Luxman should be the low ouput MC -"high" setting-63db
" I spoke to luxman and they think I should just set MC high and that’s it."
Luxman gave you correct information.
"I have read posts talking about the effect of the gain dial on MC cartridges(though it is meant for MM primarily."
The gain dial is intended to adjust to whatever cart you wish to use-MM or MC