I am strictly considering the difference of using one 800 with a single cabinet, two drivers and 800 watts vs. two 500's stacked, two isolated cabinets, same two drivers and two separate amps with two power supplies and 20% more power.
perhaps the DSP was written differently between the two very different platforms, but the two stacked might enhance dynamic power over as single cabinet design.
The cost of two EP500's is about on par with a single 800, so I am simply thinking abstractly here