I once had sibilance issues that turned out to be AC power related. The PSAudio should have helped, but maybe you need to try something else? The right combination of AC cords should also help, & a dedicated AC line if possible.
Room treatments normally are worthwhile too, but considering that you even have sibilance in headphones, this points toward the front end, those interconnects, or the preamp itself. Brass cones could also be complicating things; they can add brightness & may not be best for your rig. Try removing them altogether, or repositioning them closer together 1" at a time to add some warmth. Black Diamond cones may be better for you than brass, they are for me. Try the #3's first before the #4's, considering the sibilance. Try to borrow another preamp & be sure it's warmed up?
That brings up another issue, are you leaving at least your front end components on all the time for best sound? Cold transistors sound can be pretty harsh.
Room treatments normally are worthwhile too, but considering that you even have sibilance in headphones, this points toward the front end, those interconnects, or the preamp itself. Brass cones could also be complicating things; they can add brightness & may not be best for your rig. Try removing them altogether, or repositioning them closer together 1" at a time to add some warmth. Black Diamond cones may be better for you than brass, they are for me. Try the #3's first before the #4's, considering the sibilance. Try to borrow another preamp & be sure it's warmed up?
That brings up another issue, are you leaving at least your front end components on all the time for best sound? Cold transistors sound can be pretty harsh.