Hi Goran,
I think i have found the problem and you are right, it is the Sansui which is 39 years old. The speakers are performing superbly on my modern yamaha AV amp but the sibilance issue arises only on the left channel of the Sansui, regardless of which speakers are connected. I will have to replace all of the electrolytic capacitors in the old amp and see if that cures the problem, then work on transistors. Your design sounds fantastic and even my wife likes the look of them here in our living room. I am attaching a pic but they are not quite finished as i have to make proper stands for them. When i complete that i will send better pics.
Thanks so much for your design work and help.
Regards,
John