Bob, the mix outputs are assigned in pairs so LCR, LR+M, LR+MSum all tie up four mix outputs–same as LR + 2 mono matrix.
If you can use the second mono matrix for anything (lobby feed, fills etc) you may want to consider the mono matrix route. In my church we do what you’re considering (LR not routed to sockets, Msum to mono PA) but we only have 5-6 monitor mixes, all auxes from FOH.
LR+Msum - A stereo main mix with additional output which is the mono sum of the pre-fade, pre-processed LR mix. For example to feed a mono delay speaker or zone.
LR+M - A 3-way main mix with independent stereo LR and mono M buses. Channels can be separately assigned to any combination of LR and M. Typical application for M is main centre speaker, mono PA or bus-fed sub bass.
LCR – A 3-way main mix with true LCR panning. Each channel can be panned from left, through centre to right. At mid pan, full signal is fed to M and none L and R. Stereo channels feed the LR mix only.
Does anyone else notice FX phase cancellation when using LR+Msum for a Mono Mains sound system? We run Mono on mains and of course a Stereo Mix for a 2-track recording till we get Dante up and running. I switched over to LR+M for our last service to see if I noticed a difference, especially in Verbs. Also, if I use LR+M mode, should I assign FX module output (back panel) from Output 1 or 2? Question being is Left FX out or Right FX out considered best for a Mono mix.