I have had my MMC-1/MPS-2 setup for over a year now (using only two channels at this time) and the sibilance in the upper frequencies of the audio range still exists.
Separate from that, I also have a hum coming from one of my modules. Lonnie from Emotiva assured me that the root cause of this hum is electro-mechanical and does not have effect the quality of audio. Even so, he offered to replace the module causing the hum. In my case the hum was only audible within a foot and with dead silence so I didn't bother to tear apart my rig.
The sibilance however continues to grate on me. A flagship amplifier shouldn't sound like this. I have read great reviews of this amplifier and its predecessor (MPS-1). It could be bad modules but something tells me no since it occurs across both channels. I am afraid it's inherent in the design.
So you ended up sending yours back? I do not have that luxury now that I am a year+ into ownership. Maybe I'll hit up Emotiva support again and see what they say. Did they have any comments for you as to the cause of the sibilance?