afaik and have read, the x suffixed lineup is a bit stronger in mid-bass than the originals and has stiffer less comfortable pads than the originals as well as a slightly different look to them. otherwise, they are sonically similar.
both the 500 and 500x are going to be very anemic on bass, but the 500 more so. while good for hearing footsteps, they arent the most fun sounding headphones around.
but, the whole reason people buy them is for advantage in fps games or because they do not like bass, so moot point.