What is the cause of my sibilance problem?

I have been fighting a sibilance problem for about a year. I thought I had it beat but it is back. I've tried cleaning and treating all connections, acoustic foam treatment at first reflection points on side and front walls and ceiling, different interconnects, speaker position, and even a different digital front-end. The problem manifests itself as extremely harsh 's' sounds in both male and female voice. I am beginning to wonder if I need to have my hearing checked - I tried headphones and hear the same problem. Well, here is my system:

Aiwa XC-37M CD-changer
MSB Link DAC III with 24/96 upsampling board
B&K Ref. 10 preamp
B&K AV6000 poweramp
Paradigm Reference Studio 60 Loudspeakers
Hsu Research VTF-2 powered subwoofer
Signet center channel
Atlantic Technology surrounds
Sennheiser HD580 Headphones
Home Grown Audio Super Silver interconnects
Monster Cable subwoofer cable
Ted's Excellent Cable speaker wire (mains)
Kimber 4TC speaker wire (surrounds)
Mapleshade Brass cones, weights, and cork/rubber feet
Michael Green 5-shelf Audio Rack.

A Television, the subwoofer, and the Audio Rack sit between the louspeakers; which are approximately 7' apart and 7' from the listening position. My room is highly irregular and ASC claims that $1200 in acoustic treatment is the answer to my problem. I'm not saying they are wrong. But, having experienced the same problem with headphones (and acoustic foam not alleviating the problem one iota) I am beginning to wonder...