I've been lurking. Currently I'm playing a typical B E A D G C 6-string as my main bass, which doesn't fit my personal definition of "ERB" (even though I see it qualifies for this thread's more inclusive definition.)
But I have a Knuckle Quake 6-string on the way, in the latter stages of the build now. It will be tuned F# B E A D G. I'm a lot more interested in exploring the "subcontrabass" side of the ERB thing rather than adding higher strings -- although I know a lot of players do great things with that high range, it just isn't where my personal interest lies. But I spend so much time on the low B string that it makes a lot of sense for me to add a low F#.