I've been to several shows here. While the Myth Nightclub has decent sound and good sightlines, for the most part it is an upscale dance club which gives the place kind of a snooty vibe. During many shows a good portion of the upstairs part is roped off into "V.I.P. sections" where those who can afford it (I'm certainly not one of those) have their own little section to retreat to. Plus the security people there tend to be assholes. Also during shows drinks are on the pricey side...$5.50 for a 16 oz. bottle of domestic beer.
So whenever there are not shows here, it is indeed primarily a disco/dance club with a strictly enforced dress code. So they would not want any longhaired metalheads wearing an Opeth T-shirt.
I think a better place for the show would have been First Avenue in downtown Minneapolis. Opeth played there during the GR tour.