(Apologies for asking this, since its probably been asked before, but I couldn't find anything relatively concise)

If I wanted to keep the stock "look" of the audio system and still maintain all existing stock functionality, what are my options in terms of upgrading the audio?

Specifically, I don't want to take up additional space anywhere (for example, for a new sub enclosure). I'm open to swapping out the existing speakers/tweeters as long as they are not intrusive to the stock look. Also, it would be nice to keep the existing nav-controlled audio (I'm not sure if this is possible, however).

Any suggestions are welcomed!