Quad Z2s Owners out there?...

So I just picked up a beautiful pair of Redwood Z2s and I love em! But tonight with the bass up one of them “barked’
like it was blown but I have not had any issues with them for the last week or so since I purchased them unfortunately 2nd hand. These are designed to handle 200 watts and I bought them from an elderly man who had only had them a few months and I can’t imagine he would have blown one of them. Anyone had any experience with these?
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Is it only with a certain song or all the music you listen to? Try switching the left/right speaker cables to make sure it is not the amp. Also try to switch the ICs on the source to make sure that the source is fine.