Sibilance?

So I have an annoying issue with my bookshelfs. They are a pair of Wharfedale Diamond 10.1s and only on the right channel is there this obvious distortion present with high frequencies. It is also much more pronounced with speech.

I'm not entirely sure whether or not this is what the audio folk call sibilance or not. I have these speakers hooked up to my Yamaha RX-V375 but since it doesn't affect the left channel I doubt the receiver is the issue.

I've tried swapping out new speaker wire, used banana plugs and put rubber feet on the bottom in case it were some sort of vibration related problem. Still the problem remains.

Is this just a natural defect in the speaker or is there something else I can try? I bought these a while ago so I'm not sure if returning them is still an option.

Thanks.

How do your cones look?

Try swapping speakers to see if is a speaker vs receiver issue  

MIdwoofer cones look pristine.

I swapped the left and the right and I'm pretty sure the problem went with the speaker.

I then did you one better and connected both the woofer and tweeter separately and the problem definitely lies in the tweeter itself. And since I cannot possibly ruin the tweeter by touching it because it is always covered, I think this might just be a slightly defective speaker.