I actually called Boston Acoustics and they told me it was not possible to upgrade from the 6 speaker BA6 to the 6 speaker/100 watt subwoofer BA7 system after the car had been built. He told me it would require the dash to be completely taken apart to put in the necessary parts to upgrade to the BA7. I don't know if what he told me was the truth but I can't see why he would lie. After all, if it could be upgraded Boston Acoustics could sell all the parts in the after market.
BTW, I have had a 2005 300C fpr over 3 years with the BA7 system and I have to tell you it sounds great and I have had zero problems. If you're wondering why I called about upgrading from the BA6 to the BA7 if I already had the BA7, I was looking at an SRT8 with the BA6 (didn't want the kicker system) and was wanting to know if I bought the SRT8 could I upgrade the BA6 system.