As far as I know, the differences between the horizontal and vertical configurations of the EP600 are:
A. The placement of the logo on the grill B. The location of the pre-drilled feet holes C. The orientation of the amplifier
There's no reason you can't lay a vertically-configured EP600 on its side or stand a horizontally-configured EP500 on end, but naturally, they will look best in their as-configured orientations.