Well, let us know the results
I am not going to suggest to try one speaker, but if they do ( when ) they shut down on the QSC, try the one speaker test and see which one it is wink Doesn't Axiom pay for free shipment back and return if it is faulty?

And I think somewhere there was a list Axiom recommends for Recievers that work great with thier products, I am sure the 504 wasn't listed.

Sometimes great products need tried, tested and proved great products to give ultimate performance. This maybe an example of a F1 car running a race on gasoline and trying to figure out why he never won ?

Pioneer sc-1525
M80 HG Cherry