I am pretty sure that if you have the factory NAV that options are pretty limited. I believe the MOST bus interconnects the NAV to the head unit and the amps. There have been people who have upgraded their systems but I have never seen one done where the NAV screen is used. The NAV on the Land Rover is not that great so removal is not a big deal to most. Offroad settings would be lost though. If you do not have, options increase but almost everything is custom. The factory Logic 7 stuff is nice but very expensive to source even from a wreck. I have seen some installs of a Kenwood head unit. The installer made a custom fiberglass bezel to trim the double din to the dash. All amps and speakers were replaced as well. My only gripe with the factory system is that the AUX input seems to 12dB lower in volume than all other sources. I am sure this was done to protect the inputs from overload. When listening to the AUX input I have to remember to never use the audio on the NAV as she well shout at me when giving directions.
Check out lrrforums.com and http://www.disco3.co.uk/forum/in-car-electronics-d4-vf46.html
It is very possible to upgrade the factory LR4 audio system but you won't find the answer at Best Buy or even Crutchfield. Find a professional auto electronics shop that specializes in very high line cars and you will find your answer. There is equipment out there to interface with the fiber optic system that most audio shops haven't even heard of, because 99% of their business is simple stuff like Hondas. There may be no segment of automotive aftermarket that embodies the phrase, "You get what you pay for", more than electronics and audio.