If you want anything like a decent sound you will have to change pretty much everything....
I got a Pioneer f900bt almost as soon as I got the car and got it fitted, it sounded better that the stock HU but still wasn't great and also had to boost the signal or have the HU turned up almost to full
Just got some very nice Hertz speakers of a member on here and had them fitted this week. Front components and rear composites. I didn't want a sub enclosure so I kept the bose sub and the guy that fitted it put in a 4 channel amp for the speakers and left the bose amp just driving the sub
The sound is 1,000,000 times better, obviously would be better still with an aftermarket sub but I am more than happy.
I don't know enough to answer your technical questions, but whatever Bose equipment is in there it is all pretty weak im afraid
Now I maybe need to get a slightly quieter exhaust so I can enjoy it more