sibilence....any cures out there?

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)

Being a previous owner of the Nola Viper 1A, (which have been upgraded to the Viper 1X's), the brightness/sibilance you are hearing is most likely RF infiltration into your system somewhere upstream and the speaker is only revealing that problem.

The Vipers use high quality M-caps in the mid/high frequency x-overs so these are not the issue as I experienced very little brightness/sibilance in my setup with the 1A version. The Reference lll AlNiCo tweeter upgrades completely eliminate any brightness and are a huge step forward in transparancy/musical detail and reveal that the stock 25mm magnesium dipole domes can sound forward but this indicates a problem elsewhere as mentioned before.

RF infiltration comes from several places but mainly from poor power isolation especially from digital components and from appliances used in the home/community. If you can, try isolating your digital component(s) with an isolation transformer or a PS Audio UO. In addition, poor/budget quality IC's/Speaker cables will be a factor here as well as the Viper is a very detailed speaker and as such will reveal any shortcomings in the cable chain.

In short, the cleaner your AC mains and the level of isolation with regards to RF will pay large dividends here
and should be given due consideration.

Cheers

Before you get too carried away, do the positioning thing. Then try different speaker wires. I was surprised with relatively inexpensive speaker cables from Ultimate that took over from the very cheap temporary ones when I had moved and required extra length. Solved a sibilance issue that I was convinced was the room.