FWIW, Fremer reviews the Kleos in the January S'phile - he used it with an Avid Diva II/ SME 309. He certainly didn't experience any sibilance issues - quite the opposite in fact.
Some comments from that review:
"It's performance fresh out the box is better than a fully broken-in Helikon..."
"...cleanly rendered sibilants and shimmering high frequency transients with no hint of edge..."
"..an airy, extended top end that was decidedly not bright or etchy."
To test whether it is a cartridge/arm mismatch, perhaps you could find a friend - or friendly dealer - with an SME or similar and listen before making any decision.
Some comments from that review:
"It's performance fresh out the box is better than a fully broken-in Helikon..."
"...cleanly rendered sibilants and shimmering high frequency transients with no hint of edge..."
"..an airy, extended top end that was decidedly not bright or etchy."
To test whether it is a cartridge/arm mismatch, perhaps you could find a friend - or friendly dealer - with an SME or similar and listen before making any decision.