Problem with speaker at higher volumes - answers????

My buddy who is "internet deficient" asked me to try and find a solution to a problem he has with one of his speakers (Wharfedale but don't have the model although it would appear to be a generic type problem and I assume not indegenious to that particular manufacturer.) He is experiencing what he says sounds like a "thwap" sound coming from one of his speakers when he is playing it loudly and only on very dynamic passages. At lower volumes all is well and apparently the amp isn't clipping. I defer this problem to those more expert than I who reside here... Any possible answers???
The speaker is being overloaded. In particular if you look at reviews on , it is a common problem with the wharfedale diamond 7.1.
It's probably the woofer's (or mid-range's) voice coil or the cone "bottoming out" from over-excursion. If it's not already damaged, it will eventually destroy the coil and/or diaphram if one continues to over-drive the speakers.
Tx guys for your comments - seems logical to me. If he wants to keep the speakers would more power solve the problem or is it just a matter of reducing the volume? Changing the speakers to a more efficient model would no doubt help but he really likes the speakers...

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...
Jimbo - Bryston website advises that the 3B has a damping factor of "over 500 @ 20Hz, ref. 8 ohms. For that matter the 4B is rated the same but the 7B is "300" as opposed to 500... not sure what this means or whether these are high, low or typical numbers associated with damping???
Thats definitely the voice coil bottoming out as Jimbo stated. Better turn down the volume or destroy the speakers!
Don't get wrapped up about damping factors- the 3B should be enough. If you want to keep the "sound" of the current speakers, the next model up in that series of speakers (if available) might be the answer.