What could cause the sibilance? "cheap Dayton bookshelves". They use a cheap polycarbonate tweeter and those don't have a history of great sound if they're driven hard or their resonant frequencies are excited. I'm not saying they're necessarily bad, but they have their limitations. If they have the textile dome, it's less likely to be the speakers causing the problem.
Are you still using a box to support the mic? That's a huge problem and you really should use the OEM mic because the program is calibrated to that, not something randomly chosen.
Audyssey is trying to create a large "bubble" where the sound is consistent- since moving from one place to another changes the phase relationship between you and the speakers, this is very nearly impossible but because of the long wavelength of low frequencies, it's less of a problem than at some of the shorter wavelengths/higher frequencies.
If you shut off the front speakers and the sibilance remains, it could be the Dayton speakers (post the model, please) but if it disappears, it could be what's often called a 'beat frequency', which is the sum and difference of the signal from two speakers, reaching your ears at different times. It's more likely to happen when the frequencies are slightly different and it can be so bad it sounds like chirping.