I'm used to the traditional analogue desks, so an XR18 may do things differently, but, for me, once the gain has been set for any particular input (usually a mic...), it won't need touching, as long as the same input is used each time. I don't see how you're getting feedback if everyone is using IEM; if it's from the FOH, can the XR18 output not be reduced, instead of touching the active cabs..? There's something rotten in the state of Denmark not quite right, somewhere.
If the levels for the vocals have been set first, with the guitars, and there's no feedback, it would appear to be the addition of the drums that set it off. If that's the case, can't the drums then be backed off a little..? Are the drums leaking into the vocal mics, fully exposed if there's no singer in front of 'em..? Which individual mic(s...) are feeding back..? They are what needs addressing, not the FOH cabs. Do any of the group know what 'ringing out' is, and how to do it..? Inviting a decent sound person along for a gig or two to sort it all out would be beneficial, I'd suggest, and/or a course in PA set-up.