Only tinny sound coming from the speaker MS904

Question

I have a pair of MS904 Floor standing speakers. One of the speakers is only emitting a tinny sound. I swopped speakers over and problem followed it. Also put another set of speakers in their place and all worked fine SO NOT THE AMP. Also put speakers onto another amp and same problem. Can anyone advise me why this may have occurred. One minute speakers was fine and the next it wasn't. I am assuming that I will have to contact Mordaunt Short and return for warranty/repair