Firstly, you are right about the power sucking and (despite the flat top being even further away from the truth) there will be no square waving.
What happens if you turn the PA to full tilt, is that the incoming signal will have to be controlling the volume. The volume determines how much power the PA does (power has nothing to do with the buttons) so either way it's the same. The upside of doing it this way is that indeed you will be able to control everything from the desk (good) but the downside might be that there will be a very weak signal on the lead connecting the desk to the amp. You might pick up a lot of noise either in the lead or in the first amplification stage of the PA.
The right way of doing this is to put the desk on 0dB, put the PA up to the level you think you will need plus a little bit more, and then back off on the desk until you are happy. During the performance, you can still push the desk a little into the +dB side without having to fiddle around for the PA volume knob, which is a lot harder to quickly turn up the right amount (IIIII feedback!).
The closer the desk is to 0dB right around, the less side effects you will pick up. Avoid pushing any button or fader on a desk above +6dB if you can, most of them do not sound so lekker in that region. Avoid putting very small signals through the power side of the system, since somewhere the signal will get amplified a lot again.

Maybe read up on Alan's article about gainstaging.

+1 on everything GH said above. Keep the amp set to 0dB and run it from the desk - that's the way pro gear is made to be run. If your max volume for the venue needs to be lower than your speakers and amp produce at full tilt, then lower the sensitivity of the amp to compensate.

Underpowered amps kill far more speakers than overpowered ones - keep an ear out for distortion from the amp - that's the real speaker killer as it makes the voice coils overheat and burn out.

Thanks guys, suspicions confirmed.

Alan Ratcliffe wrote: Underpowered amps kill far more speakers than overpowered ones - keep an ear out for distortion from the amp - that's the real speaker killer as it makes the voice coils overheat and burn out.

Yeah we actually got more powerful speakers with the view to eventually running some bass bins as well, at which time we'll have to upgrade the amp. We're careful with distortion, we'll keep an ear out...

Gearhead wrote: Firstly, you are right about the power sucking and (despite the flat top being even further away from the truth) there will be no square waving.

May I ask why flat-topping is even less likely than square-waving?

VellaJ wrote: May I ask why flat-topping is even less likely than square-waving?

Music is between amp and speakers an AC voltage. The positive swings are (over time) the same amplitude as the negative swings. Your solid state power amp makes this out of a 'symmetrical rail': a set of positive, ground and negative voltages. The positive is as much voltage to earth as the earth is to negative, in a proper amp power supply. Meaning, if you want to push out more power and the amp runs out of volts, it will do so at the positive side as much as on the negative side. If this happens to extremes, the signal runs from positive power supply voltage, very quickly down to negative power supply voltage and quickly back up. This is what they call square waving, and it ruins speakers tweeters first.
You probably assumed that the music is represented by a positive-only signal, so that if the amp runs out of volts it flattens the tops. the point is that it flattens both the positive tops and the negative tops, which looks like a square wave.

Gain stage the signal upstream of the amp to 0db and turn amp knobs to desired level (which could easily be full, don't blow the speakers, stop when you've reached the right volume or when the speakers reach their limit then back down a touch). The knobs are not volume knobs, if they are set low it just means the desk must drive a higher signal to clip the amp. Gearhead is right.