what causes the sss in words like 'thisss'?

I run apple lossless codec -> airport express -> recently purchased musical fidelity x-dac & tube buffer -> rotel 1056 receiver -> Quad 22L floorstanding speakers . I have also tried a cheap cd player source but no luck.

So what causes those annoying sss sounds in words? Source? Amp? interconnect? I'm considering returning the musical fidelity stuff and spring for a naim CD5 and nait 5i combo. Thoughts?