Does anyone have suggestions on how to improve the sound quality of the factory 'premium' system. Will changing the speakers and sub alone be enough for improvement, or will I need something more? Also, is there a way to convert the XM portion of the factory system to Sirius and keep the display as is?

 

This has been covered countless times, but I will go over it briefly. Please search for more detailed information.

Simply upgrading the speakers will only give you a very slight improvement. If you do upgrade just the speakers be sure to find some that rated for less than 50w, the less the better, and efficiency rating of 90db or greater. Getting quality speakers and providing them with enough power will yeild much better results, but you still have the stock HU holding you back. The stock HU has no controls, and a built EQ to make the stock speakers so boomy, so you can't get a flat signal from it. If your going to keep the stock HU, and plan on having good sound I highly recomend looking into a JL cleansweep or similar interface, also a nice crossover network will give you more control over your system with the stock HU. A quality aftermarket headunit will give you many more controls, and allow you use a processor to give you even more control.