sibilence....any cures out there?

Replace the capacitor to the tweeter with some vintage paper
In oil ones.

Agree with the treat the room posts, especially first reflection and bass traps.

Treating the room will not fix the direct sound. The issue is an elevated mid treble, not an "ultra detailed" tweeter. (Does the Viper use ribbons?) Only real solution is to flatten out the treble with equalization. You'll need a calibrated SPL meter and test tones.

Best and cheapest way is to toe your speakers in til they converge just in front of your listening position. This usually helps in imaging too. It's easy to try anyway.

As with all audio performance, assuming nothing is broken, 95% of the sound you hear is your speakers and your room.

Sibilance is in the 5-8000hz range which means it's in the tweeter.

Start by playing around with speaker placement, and also try changing where you are sitting. Moving a couple of inches can change things by 5-10db.

Next is to figure out what is going on with your room. Have you taken a thorough analysis of your room's acoustics? If not, time to look into room acoustics.

Alternatively, change your speakers back.

No cable change will fix your problem, (unless you have a broken cable)