I agree with Adveser somewhat, but you said the majic words right here:
"I forgot to mention about the speaker they used for that event. It was a very old one already."
Depending on how old the speaker system is, some of the 'old school' midrange drivers started rolling off 6.5khz. It wouldn't matter what kind of high dollar(full freq. range mic)you used, like Adverser said. The speaker system would not be able to reproduce it no matter what mic placement. That's why the violin probably sounds like crap when they mic it.