your mini-disc probably was not capable of handling a +4 feed from a soundboard. that signal is meant be fed into a little higher end recorder that is capable of handing that sort of signal. with proper attenuation between a +4 signal and your deck, youll cut the signal level down to something that the input of your mini-disc can handle.
perhaps the soundman was trying to get you to upgrade your recording unit when he told you that he couldnt cut the signal, because in most cases, he could have cut the signal significantly, maybe leaving you with a listenable tape.