It wouldn't be a stretch to modify one to use 6L6s. The one above (on Reverb) looks like it was a special order (and probably with an output transformer optimized for 6L6s). Still, as the owner of a 20th Shiva with KT88s, I couldn't imagine the amp being any better than it is. AFAIK, it was designed around the KT88s, like the 20th XTC was designed around 6L6s, or the Twin Jet was with KT88s. I also have an Uber and it's had both KT88s and EL34s in it. I think it sounds better with EL34s, as was originally intended.
BTW, it's not that I'm against tube rolling and other optimizations. It's just that for the differences between 6L6s and KT88s, you'll see just as big a difference, if not more so, between different manufacturers' KT88s, or different manufacturers' 6L6s for that matter. It's just not that big of a difference, IMHO. You'll find bigger differences in changing 12AX7s and even bigger differences between different speakers. I also think that if an amp's just not quite what you're looking for, it's better to find another amp that is closer than it is trying to make something into something it's not.