As far as I know there is a lot of debate on whether you can destroy speakers by over-powering them. In my opinion, this could happen if there is a huge mismatch between the output wattage and the speakers’ maximum wattage. However:
This may only happen if you pump up the volume to max. If we are talking about 2x50W I would rather worry for my eardrums, not the speakers.
When over-powered, speakers will distort and you will be aware that you leave the “comfort zone” of the speakers.
Except for short spikes, the amp cannot provide more power to the speakers than provided by the power supply. For example, if your 24V power supply can provide 4A of current, the amp cannot provide more than 100W to the speakers. Therefore, unless your power supply is really overdimensioned you should be on the safe side.
If I were in your shoes, I would not worry about this 21V vs. 24V thing. Rather, I would be happy to have a 24V power supply to have some reserves. I power my arylic 2.1 amp with the used power supply of a notebook (21V/4A) and I’m still angry that I could not find a 24V/4A one .