I've had the same issue, usually with loud piano passages. Others on the forum have stated similar problems usually with the 3.5 midbass. I have contacted Infinity several times and get the usual "we don't receive any complaints about that speaker". Guess I will try snail mail. You can try this link, it looks like they have parts.
http://www.harmanaudio.com/search_browse/partlist.asp?BrandId=INF&MarketId=HOM&Parts=PMTS-TOWER
http://www.harmanaudio.com/search_browse/partlist.asp?BrandId=INF&MarketId=HOM&Parts=PMTS-TOWER