I believe you can. I've seen people w/ the LS460 swap out their premium speakers/amps for the mark levinson ones. So don't see why not. I think the problem is finding them. As for the IS vert, they are quite rare to see the ML system in this car. When I was looking for a vert, my first was to have the ML, but could not find it anywhere.
If you get the speakers and amps, post some pics.. If you decide to go that route.