I wish I had the time to fly over in order to hear some of that fantastic fiddling ...and congratulations on the excellent "brief" you've posted.
If only more users took the time required for the huge effort involved in setting out their requirements as clearly as you've just done....
To my mind, your "key" words (that seem to almost jump off the page) are "gymnasium" and "electric bass & keyboards on occasion".
"Gymnasium" = reverberation. Lots of it. (But you didn't need me to tell you that.)
Also probably known to you, but maybe less obvious to other readers, are the demands that "electric bass & keyboards" place on any reinforcement system with regard to the clarity required for all performers to be able to hear each other, ...without which it's impossible for them to properly become an ensemble, as distinct from a struggling collection of individuals.
Combine all of that and we are faced with the need to plan our speaker placement very carefully, so as to reduce the number of points from which our reinforced sound radiates to the absolute minimum, as every point of radiation we might add in seeking to "help" will only combine with the venue's unfriendly reverberation to further confuse our sonic image, ...a classic (if counter-intuitive) case of "less is more".
(Mind you, that sort of thinking is the closest thing to music when it comes to the ears of our "accounts department".)
Given both your relegation of the "local park" to secondary status and its far more "friendly" acoustic profile, I'll concentrate mainly on your gymnasium rig, but in saying that, I heartily endorse your "backup philosophy" in respect of the possible purchase of a second M20d.
Speaking of "backup", it's worth bearing in mind that your consideration of a little extra expense for the "t" versions of StageScape speakers may actually yield savings by allowing you to re-think the need, the timing of and the configuration of extra purchases to fulfil those "park" and "backup" requirements, as you'll see from the "10-Input Connection Diagram" at the bottom of this page: http://line6.com/sta...e-l3t/resources.
Now, back to your gymnasium project.
Even given its 1,200 attendee scale, my feeling is that your gym system will probably require no more than ONE pair of L3 enclosures as its principal sources, the "secret for success" being the physical and electronic details of their combination with a further pair of L2 enclosures and a single L3s "sub", the "electronic details" of which would be an absolute nightmare in the absence of Line 6 linking, but instantly resolved with it.
Accordingly, it's the physical details that form the main substance of my recommendations, the first of which would be to get your main pair of L3 boxes as high as you can, to cut down on the clarity-destroying and feedback-inducing standing waves created by floor placement firing straight up to reflect from the roof of your gym back to its floor, then reflecting again from the floor back to the roof, then again from the roof to the floor, ...a truly vicious cycle, if not circle.
I've had several pairs of these back-savers for some years now http://www.amazon.co...n stage ss8800B that I use for exactly that, and I'd heartily recommend one pair (of those or something similar) for your L3 enclosures, and another pair to elevate your "foldback" pair of L2 speakers.
Why have a "foldback pair" when you say that "monitor requirements are minimal" ...and why elevate them ?
Well, I honestly don't believe that our FOH pair by themselves will allow our performers enough sense of each other to properly interact and the classic "wedge at the feet" foldback placement simply will not work for this application.
Not only might we need up to half a dozen floor-bound speakers to cover the width of our ensemble, but the sound from them would arrive at the players' ears before also arriving from our main pair, in addition to suffering the standing wave problem I mentioned earlier.
The result ?
A jumbled, incoherent mess.
It's been my experience that the best solution when playing in these reverberant "boxes" is to locate our fully elevated main pair at a location that is NOT an exact quarter or half "gym length" downstage (towards our audience) from the wall behind our performers, (something like "three-eigths of a gym-length" is generally best) but also roughly "an eighth" of the width IN from those equally reflective side walls, with our foldback pair co-located immediately behind them, (maybe not elevated quite as much) but facing the opposite way, towards our performers.
That co-location of our foldback pair means that, in effect, all of our sound is radiating from the same points in space, granting a coherence that gives our performers a real sense of their overall balance, as well as automatically placing their foldback in "dead spots" in terms of the pick-up patterns of the microphones we're using for them.
Another benefit of that co-location is that the subsequent "time alignment" allows our main pair to do most of the work, whereby our foldback pair can be operated at surprisingly low levels, so that far fewer undesirable artefacts arise from our unfriendly "chamber".
(I generally set that level by playing a known recording to FOH and then slowly "easing" the foldback up to a point no louder than where it just "creates" the required "image" at the performance position.)
Why only one sub ?
- Gymnasiums invariably "boom" in the bass region anyway, so the extra bass power definitely isn't needed;
- Experimentation will often reveal that corner or "centre stage" or "under stage" deployment works better than would be achieved by the placement that would be forced on us by long poles being used to elevate our mid-tops above a pair of subs;
- We are the ones that have to truck and lift the darned things ...and
- The expense.
Anyway, that's my "take" on "the required scaling of a system for our main event" ...and here's to many more of them !
All the very best from Down Under,