thyname,
The definition of sibilance is (from www.thefreedictionary.com):
sib·i·lant Pronunciation (sb-lnt)
adj.
Of, characterized by, or producing a hissing sound like that of (s) or (sh)
Sibilance is not found in the sound of a clapping hand. Sibilance is only in voices in a recording. It is part of speech, not part of an exact sound frequency but instead varies by individual voice. Sound frequencies in general can be affected by their environment but the S will always be in the English language no matter how high or low it is pronounced by the human voice.
Often cymbols are deemed 'sibilant' because the sound they make is similar to a sss type sound one could pronounce, although it is not the same. The cymbol sound may however cross into a frequency range similar to that of most human voices on average.
I knew a guy once who could make trumpet sounds pretty effectively, but it was still no where near a real trumpet.
I have to agree with Jack. It is likely the JBL speakers have a wicked dip or huge hump which is masking a common range of vocal sounds or instrument sounds making the harsh notes seem less harsh. Turning down the treble knob does the same thing, but you lose alot of detail with such methods.
An A/B test with both speakers would be more useful in determining if this is such a huge difference b/w these models. Going by audio memory is notably unreliable and therefore near impossible to answer questions like "why when i added this component did such a sonic character disappear/appear...etc.".
