help with speaker problem

I have a problem...i notice it only on piano music, it sounds like the tweater of both my speakers is distorting, simmilar in sound to when a speaker clips, only not nearly as loud, but audible from my listening location...it also seems to be frequency dependent..ie. only occures, or occures to a much great level within a certain frequency range judging by the piece of music i was listening to when i really noticed it around the C above middle C on a piano give or take a bit.

when i first noticed the problem in my left speaker returned the speaker and they replaced the tweeter with a new one, but the problem persists and is noticable in both speakers...

my system is

totem arros
nad c350
kimber 8pr cable
eichmann express 4 interconnect
dennon dcd 3520 cd player

any idea what it could be...I wouldn't think three tweeters could all exhibit the same problem unless its a design flaw with that model of speaker (unlikely)... i thought mabey the piano notes happened to fall right around the crossover point of the speakers and that somehow caused the distortion...
im biwiring with kimber 8pr, could low frequency sound somehow leak into the tweater...
you guys are really smart...help me out!!, i dont want to just get the tweater replaced again cause i dont think it'll fix the problem, and i dont think that totem makes such poor quality part that three tweaters would all exhibit the same problem, and i doubt id be able to get my money back from where i bought them, as they're about 8 months old
it definately the tweeters though, ive tried everything, including hooking them up to an entirely different system, so i know its nothing in my system causeing it