Let me ask you this; is your sibilance problem something that bugs you on all vocal material you listen to? On the other hand, could you be focusing on only a handful of selective offending CD's? I ask because I know too well the obsessive nature of most audiophiles, myself included to some degree. There are certainly specific CD's that I know to have problems with sibilance - so much so that I wonder how much of it is in the recording and mixing. For instance, I can play Casandra Wilson's, "New Moon Daughter" or "Blue Light Til Dawn", and be sure that no matter what speakers either is played on that there will be some pretty hot "S's". Certainly it can be worse on some systems than others. In the case of my SR17's I can sense that hot 'S', but I've heard it much worse and it has never occured to me as offensive on this setup. It is no worse than on the two other speakers (neither using Esotec tweets) in my home. Same thing with Diana Krall's, "Live in Paris". Now I could see if you were focusing on challenging recordings like those, and nothing else, where you could get caught up on focusing on S's. Then you'd be missing the rest of the music, and that would be really annoying. Have you tried any other speakers side-by-side on your system, or taken your SR17s to play on a friend's system with similar offending program material? Before making serious changes, especially with gear you seem to otherwise like, I'd want to make sure that it was, in fact, the specific gear I was considering giving up that was the offending link in the chain. The other component I'd carefully consider is your front end - have you tried anything different?
Just a thought.
Marco