Theres a detailed description on a few options on Pelican Parts website below.
http://www.pelicanparts.com/techarticle ... nstall.htmThe book they mention "A 101 project for your Boxster" has all the above in & is worth owning.
My 03 Boxster has a Parrott MKi9200 kit in it which is ok but it uses its own amp thus by passes the OEM one so whilst it is really very good at making phone calls/using ipods, memory cards, USB sticks it doesn,t give the same punch/volume of factory fitted unit. Like the Denison kit you mentioned expensive too making replacing OEM amp & headunit a viable option. Mine was in the car when I bought it but I wouldnt choose it my self & will be replacing the original head unit with a Mechless Sony head unit I have got. My choice would be too replace the OEM head unit & amp but I havent looked how easy/difficult this is on a BOSE set up.
HTH