I like 50% for the master volume slider so I can boost or trim to equal degrees.

Matching input preamps to Montage outputs will be a manual process guided by meters, clip LEDs, and your ears.

Good advice.

When sending the Main L&R output of the Montage to a Power Mixer (such as the Dynacord Powermate) should I set the Main L&R Output Gain to +12 dB, as this would provide the highest signal to noise ratio from the Montage to the Power Mixer?

That's a no, you will need to determine if you need more gain from the Montage. It would be highly unusual to need to just set it to +12 to start. You would do so because you "know" you need it, not before. How do you "know"...

It's not that it's stupid question (I was always taught, the stupid question is one you don't ask. Asking is the smart thing to do) what you want to learn is each device in your audio Chain should output enough signal so that the next stage has enough to work with... but can't get too much. This is where your meters and peak lights can guide you.

You want to send enough signal from the Source (Montage) so the INPUT STAGE of the mixer has enough signal to do its job. You want to send signal from the Montage that gives a good representation of your range, soft to loud. And make sure at your absolute loudest peak, the peak warning light is still able to flicker on/off, without staying steady. That peak is always several dB short of clipping. So if it can still flash on and off at your loudest you can then be assured your mixer input is happy. And will not distort.

The next stage in the signal travel is through the channel FADER. Once you "gain staged" the Input at the Input preamp stage, you can now raise the FADER and send the signal to the main outputs of your powered mixer. Raise the Main output to taste. Yes... use taste. Loud enough so people can enjoy your distortion free performance.

When setting level... You are feeling for the ceiling... finding the upper limit, and making sure you will not bump your head. A famous 7' basketball player was giving a tour of his home, and had this huge 8 foot bed...a gigantic pool, etc., etc... just enormous... the house was built to accommodate him... all but the doorways. They were all standard height. When asked why, his answer was priceless. "When I enter room, I'm used to ducking... that's all I need is to start forgetting to duck." 🙂
You must know your limitations...

The morale is know where the limits are, test the loudest you are going to possibly play... ensure that this level cannot clip the system. Now you can run around without having to worry about distortion... bumping your head.

Second question, how high should I put the Master Volume slider of the Montage? Should I put it at its maximum value or could this create distortion and maybe damage the montage output? Apologies if this is a stupid question, but I keep the slider at about 75% at the moment, for fear of these things.

Again, this depends on your situation, it is often both your scientific and your subjective Output. You should be able to raise your volume 100% without overloading your mixers input. So verify that your 100% does NOT, and cannot distort your mixer's input stage.

I would then back down a bit so you some artistic room to work. But the idea is to setup so that each device can send to the stage enough to satisfy its need for input... without being able to distort it.

that's all I need is to start forgetting to duck.

Ouch! 😀

Thanks for the explanation, the concept is a lot clearer to me now. I don't think I'll forget that quote anytime soon.:)