Everything I could say has been said already--except that if people got angry with me for all the things I've failed to pay proper attention to over the years, I'd have been beaten into dust long ago. So I'm slightly more forgiving of the original poster. It probably never occurred to him that he could cause damage--so, although we know better, it's not entirely fair to blame him for not thinking about that.
The risk is real, though. There was a whole article about this in the Journal of the Audio Engineering Society a few years back. The line outputs of most professional audio equipment simply can't handle 48 Volts, and that's not likely to change. If it were relatively easy to add effective protection without compromising the sound quality and driving capability of the output circuits, I'd agree that the sound board and other audio equipment manufacturers should all do it--but such is not the case.
So it is, and will remain, the responsibility whoever connects anything to the (actively driven) outputs of anything else to be 100% certain that no powering is applied to those outputs, unless they're the outputs of something designed for that type of powering.
--best regards