tstarn06,
I too hear the sibilance when using the stock single flange tips with my CX550 (the new CX95). Having read extensive reviews on the CX95 and their sibilance issues, I completely bypassed the stock tips in favor of those eBay bi-flanges when I took them out of the box. In all honesty, the tips that Sennheiser packs with the CX550 (which also includes the harder bi-flanges that also come with the IE series) are rubbish to my ears. The eBay bi-flanges reduce almost all of the sibilance, and really open up the sound, especially the soundstage and treble clarity.
Of course, this post probably doesn't help since I already have around 25 or so hours on my pair, so I couldn't tell you whether the sibilance is greatly reduced with burn-in.
astroid,
Some people are sensitive to sibilance. Personally, I don't hear much sibilance at all with the bi-flanges, so different tips could make the difference.