I'm refinishing some old speaker cabinets and noticed they were out of square. They're reasonably sturdy, but if I push the top sideways while holding the bottom (shear force), I can bend them square. In case it's relevant, they're just four boards nailed or screwed together... I can't tell for sure because the fasteners have been covered with wood filler.
I had my wife hold one of the cabinets square by pushing the top sideways while I installed angle brackets on two of the four corners. But when she let go, it immediately went back to not being square. In other words, the two angle brackets weren't strong enough to hold the cabinet in the desired position.
I'm not sure how to get these cabinets square without disassembling them... try more brackets? I have plenty more and could put two in each corner. But I don't want to put a bunch of holes in the cabs if it's not likely to work. Or use beefier brackets? I'm not sure where to find something like that. Or maybe some other way?
By the way, if I have to disassemble them, I think I'd just leave them out-of-square and tell people they were designed that way to add to the tone.