I don't see any measurements stating the >500W is RMS. Only that it's capable of hitting >500W with a specific THD+N value.
I would never be using it at 500W continuous, nor full bandwidth (i.e. white noise). The Magnepan 1.6's are only rated at 250W power handling, but if I switched the ST-500 over to my Onix Reference 3's then I might give it transients of 550W while watching movies.