Yes you can simply add a sub and amp if you only want more bass. You don't have to replace the whole system but that is advised since most bass upgrades overpower the door speakers.
sorry...but this is just wrong advice.
#1 - any high level input device is just trash compared to a true low level input and distort way more. taking that out of the equation.....
#2 - the B&O tuning starts to roll off the low frequencies when you try and get any volume out of the system. Whether you take the full signal off the rear doors or the sib signal off the factory subwoofer......you are getting a signal after the B&O amp and its tuning ..... there is no way to get accurate low frequency information at higher volumes. So to get any decent bass, you have to run the gains of your new sub amp very hot, which then makes your subs over power the rest of the systems. Unless you just want BOOM POW....dont design a system like that.
Audio control bass restore is just electronic guess work and deviates from the actual recorded information in the track. If you care anything about good sounding music, you can hear this gimmick and it sounds like crap.
If anyone tells you a high level input device like an LC2i is "just as good" as a device like a NAV TV Zen A2B....they are lying and just want the sale, or dont know any better. you need to decide for yourself.
I have 35 years of experience. While things have changed a great deal over the years, one thing remains constant. High level inputs will always suck.