Are you sure the cymbal was actually in the mix throughout the song? There are an awful lot of variables here. Have you listened on headphone to confirm that they just haven't gotten lost in the rest of the mix? After all, if you heard those frequencies in the beginning, your system can obviously reproduce them.
First thing I'd say is turn off MCACC if it's turned on. I'd also see if you can hear the soundstage you want with a well mastered CD, playing only stereo. that may help you isolate if any of the other speakers or particular surround modes are the weak point (which I find to be more likely than the M80s being the culprit.
I didn't do it, no one saw me, you can't prove anything.