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GR98 clear: https://youtu.be/nc7fZYMlOzg
Even though Istvan's Paired Blades were untouched in 2.6.1, the rest of the WW setup was greatly buffed, so the basic build got automatically buffed as well. Istvan's variation of WW gained strength around a year ago but unfortunately i never had the time to create a proper guide for it, even though i played a significant role promoting this setup among barbs. Barb community is always very skeptical and conservative, even though some players actually played Lightning Istvan's, it was pretty hard to convince them to try multiplicative damage of Istvan's over multiplicative attack speed of Bul-Kathos.
In the end the build took-off and had a great success, and it still tops the leaderboards. Istvan's, while being slower, provide solid single target damage as well as extra armor.
With the recent buff to Bul-Kathos's, however, the mighty weapons might REGAIN their glory once again. I cleared GR98 with both variations and found literally no gap in firepower.