Most phones normally comes with a non-standard power connector, so people can't cause problems with alternative chargers. Unless they buy a cheap chinese knock-off.

And a large percentage of phones have a standard USB connector, so the user either get standard 500mA charging current or (if the phone realizes that it is connected to a charge port - standard or their special variant - it switches up to some more reasonable charging current). But people doesn't manage wrong polarity or voltage for USB charge ports either.

There are lots of nice switched supplies today that can handle large currents with small components. And that can either supply step-up or step-down or sometimes both. So you might have a device that runs from 5V to 30V without problems. Potentially allowing you to add protection at 20V and have a 10V headroom internally.

We make equipment that could work from 7V to 60V. We market them for use between 12 and 30V and have overvoltage protection stepping in at 35V. People use them in rough environments but we still have very few complaints about problems caused by supply voltages. We have a few customers who have tried to route 230 and 400V mains power through some of the circuits and the are of course limitations to what designs can handle but the products normally works or gets back with significant scorch marks where multidigit amps have been misdirected.

What power connector do you have? Lots of people have lots of power supplies laying around, so it really isn't easy to pick and match perfectly. I have hundreds of wall warts in different sizes. Accidentally taking the wrong one do happen. That is a reason why the round DC connectors with a pin in the center have a standard where a larger diameter is intended for higher voltages. So as long as the wall wart have overcurrent protection, most people manage quite well even when they pick the wrong transformer.

One thing with your onstage equipment is that if they get a broken power adapter, your customers may be absolutely desperate to get their equipment working because they have to perform at a fixed time. They can't just tell their audience: Ooops sorry. Our $15 wall wart broke and when we tried a $10 wall wart we found in a bag our MIDI equipment refuced to play - and there is no store available where we can find a 9V adapter rated 1.5A. So, sorry if you did drive a long distance, but we hope to play here "again" next year.

Do think about the desperation if something happens with that 9V transformer.