Socaste, Logic, not hands on experience, leads me to a few conclusions based solely on what you have posted regarding your equipment and things you have done to solve your problem.
To review, as I understand it you have sibilence with any source materiel but it is worse with vinyl. The sibilence remains without regard to the amp/pre-amp or tubes you are using. You have ruled out room/set up acoustic related problems. What's left? Your speakers of course. Now that could be your speakers themselves(probable) or their interface with your amp (more likely, except you have tried other amps).
Perhaps it would be meaningful to review how your problem first manifested itself.
When did you become aware of the sibilence?
When you first introduced the speakers to your system?
When you first introduced your amp to your system?
When you first introduced or modified your TT system?
You have sort of dismissed the suggestion that your speakers and your amp might be a mis-match because you know folks who have them and are happy with them, but you do not indicate that you auditioned either or both in your home before your acquired them or that you auditioned them at the homes of the folks who were supporting the combo.
As your thread now stands the only common denominator for the source of the sibilence which you have not ruled out is your speakers. FWIW, some years ago, I bought some highly regarded and extensively reviewed, speakers which I auditioned in a showroom before purchase. I was a neophyte and didn't think about amplification issues. I spent the next two years looking for the right amp (my speakers were sibilent as hell!) and I finally discovered by accident the amp used by the manufacturer to voice the speaker. Bought the amp and the speakers were every bit as good as I had been told, except of course, by then I hated them!
Bottom line, call the Manufacturer and ask them what amp they used to voice their speakers. While most won't tell you, you might luck out on this one. :-)
