A touch of brightness

[thumb5]

This might sound like heresy, but: have you considered using the tone control to apply a bit of treble cut to help reduce the sibilance?

You can use the configurator to set the frequency at which the treble control operates - I'd guess you might want to start with something around 5 kHz.  Then you can select the amount of treble cut using the remote control (documented in the manual for the 8.1 firmware).  It will probably take a bit of experimentation to discover what combination of frequency and cut best controls the sibilance.

I haven't done this myself so I don't know how much detail you might lose - the cure may be worse than the disease.  Anyway, there's nothing to lose by trying it, apart from time.