I'm a big fan of keeping things looking discreet as well. It would be very hard for me to have some third-party audio receiver in there. I think I'm just going to stick with what I have, and as many others have said, just enjoy the Boxster's engine noise. I have an amazing home theater system that I can do my serious audio listening on. I just thought it would be cool to occasionally be able to crank some Rage Against The Machine during a long day of driving.
In other words, I have decided not to upgrade, haha... it's too complicated and expensive with the Boxster. Thank you to everyone who posted or was willing to help.