2005 Nissan 350-Z stereo upgrade

To keep the factory head and ungrade from there is going to be your best bet. Use the Audio Conrol 6XS to integrate between the factory Bose head and the new amplifiers. The 6XS is advertised as a crossover, however it is alos a highly advanced line output convertor + it will give you the ability to control the level of your sub from the supplied bass remote (which can be taken apart and installed into the dash). Add a good 5 channel amplifier to the mix next. I happen to like the tried and true Eclipse PA5532 if you can find one. Next a new speaker system all the way around is going to be in order as the factory Bose speakers are impedence optimized to the Bose amp. So want to change one thing, you must change them all. Consider good speakers like Boston, Focal and Eclipse. As far as a sub goes, if you are after good tonal response with the ability to get a little loud, a single 10" or 12" driver in a small sealed enclosure will do the job. Again I will always recommend the tried and true Eclipse aluminum series as it has a highly predictable outcome for the paying customer.

Now if you want to blast yourself out of the car, we need to do things a little different. Confirm if you are after laying waste to the neighbourhood or want good sound.