Going off of RMS power is the true fair way because if my amp is rated at 1500 RMS then it is pretty close to any other amp doing 1500 RMS
(RMS is a mathematical formula which is pretty hard to fudge or lie about).

If one guy runs his amp below the recomended and rated impedience level and gets more power out of it then if he doesn't cook it good for him but that is not the only thing involved in making an SPL meter move

The classes are broke down with the consumer in mind and to try and make it as fair as possible between the competitors in the same classes.