The speaker is being overloaded. In particular if you look at reviews on www.audioreview.com , it is a common problem with the wharfedale diamond 7.1.
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From your previous post, you mentioned that the amp wasn't clipping and, assuming also that the amp wasn't distorting, the amp probably is not the problem. You might try an amp with a very high damping factor as that kinda keeps the woofer under control, but more than likely, you are already over-driving the speakers. Maybe the next larger size speaker in that series would help. A powered sub and a separate active x-over could help, too, but check your bank account first.
What do you have for power?
Jimbo - the amp is a Bryston 3B-ST and while the Wharfedales are relatively inefficient ( I think somewhere around 86 db SPL rating) I would think the 3B with its ability for 200 w/ch @ 4 ohms would be enough - As I am unfamiliar with its "damping" ability I cannot confirm whether this is an issue. I'm used to Maggies and have never experienced this type of problem in the past and therefore quite in the dark...