Hi Daniel,
As I wrote in that post and here back in January 2013, I, too, was worried about the hiss level I was hearing from the X32 through my shop's UPJ-1P's until we connected the APB Dynasonics House Rack that we'd previously been using with great results through all kinds of systems. The APB's hiss level with the master down was quite a bit less than the X32, but when it was at unity it was massively louder than the level from the X32 either at unity or off. That made me stop worrying about the X32's noise level. BTW, nine X32's all were the same, and too much hiss has never been a problem since.
In your case you say it's fine with some speakers and bad for these Mackies, so, yes, there is clearly something different with the Mackies. You say there is no volume adjustment on them? No mic (-10)/line (+4) switch that is currently set to mic? Other Mackie PA speakers that I've seen have that feature, funny/good if that model doesn't. Although it solves a minor problem (no mixer, one mic in use), it has the ongoing potential to create a massive one.
If the hiss is truly unbearable you either have a bad console (unlikely since it's fine with some speakers) or too much gain in the Mackies, and my money is on the latter.
Good luck, and keep looking.