You never mentioned a bass problem, which is what corner and ceiling corner treatments would address. My fave(Auralex), Applied across the wall behind your listening position, will resolve your issue(slap echo/excessive upper mids resonance), ie:(http://cgi.ebay.com/Auralex-Wedgies-Acoustic-Foam-Studio-Soundproofing-24pk-/370320703651?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item5638d580a3). There are also lesser priced knockoffs on eBay, if you type, "acoustic foam" in the search bar. The 2" thick wedge stuff will be right for your target freqs. Find the side wall/ceiling reflection points and apply some absorption there, to tighten your sound stage and improve image focus.