Speakers don't care which way you wire them, but if there's a set of them they all need to be wired the same way. If your Dually has one driver wired one way and the other driver wired the opposite it'll still work, but the signal coming out of the two drivers will be out of phase, which will cause cancellation and greatly reduce volume. It's entirely possible for it to still sound good, but just not have the volume output that it should.
So plug in a speaker cable with a 1/4" end and touch the TRS plug to the terminals of a 9V battery, with one terminal touching the tip and one touching the sleeve. The speakers will make a pop sound and jump outward. If they're in phase, they both push outward. If they're out of phase, one will push outward and the other will pull inward. If that happens, it's a simple matter of swapping the leads on the one of the drivers.
It's an easy enough thing to check and could be causing your issue.
Not to keep piling on, or second-guess your conclusions (you ARE dealing with a stupid amount of guitar amp volume after all), but I'd try the battery test and see what happens.