I had played a dingwall 6 before I placed my order, yes. What I did not do is play one for a typical 3 hour gig standing up. I think my main issue is not the playability, because it plays so well sitting down. My issue is more the fact that 6 strings have wide necks that make it difficult to keep a straight wrist when the neck widens around the 5th-7th fret, as @AB Nate alluded to.
Ironically, I played for a solid 1.5 hours yesterday, sitting and felt perfectly fine. I do think it might have something to do with just general fatigue by the end of the night. My band plays our sets medley style so I'm literally playing for hours at a time while singing and moving on stage. I think I may just have to use a different instrument that's a little more forgiving regarding technique for those gigs and save the 6 for home practice, recording and shorter/sitting gigs. Thanks everyone for their input and help.