I really don't know what to recommend, because it sounds like you have already done the majority of what I would suggest.
Do you have the litter boxes spread as far apart as possible? I heard spreading them out is most beneficial.
I would think that this is anxiety. That is typically the number 1 cause of cats urinating outside of the litter box. *Especially in multi cat house holds*
When I hear terms like "hipster" I think, who told cliques they could leave high school??