Are you sure that Xlvl does not work on cue outs? I´ve never actually tried it, but I´ve read that Xlvl only applies to main outs, hence it is recommended to swap the roles of main out and cue outs when wanting to use the crossfader for fading in/out external send/return effects.
Xvol won´t work, because it is pre effects, it mutes audio before going into the effects and thus also to main and cue outs.
For example, this thread mentions it.