If Underworld did separate gigs.......

[darktrain]

What about a split set w/intermission in between? Do an ~80 minute festival type set, and instead of an encore, break for 30 mins and then reconvene for the "chill" set. If Karl being inactive is an issue, a live jam painting or other art installation going on could alleviate that, though I'm sure some curated tomato visual magic would work, too.
I'd personally love a chillout set from them, but it would have to be heavily advertised as such or the crowd could very likely be let down.