its called bass attenuation , a similar function to "LOUD" on aftermarket stereos, the high notes and low bass notes are increased at low volumes, and increased at higher volumes.

Its hardcoded into the bose headunit so theres really nothing you can do about it. The bose system is designed for clean sound throughout the volume spectrum and the bose sub will clip at higher volumes unless it is attentuated.

You could replace the headunit but the sub will probably clip once you reach the volume it currently slopes off at.

As for replacing the sub, the bose sub has a very high sensitivity and is usually an uncommon ohm load. if you use a load lower than it then it will burn the amp out. if its higher you will lose a lot of power and it will sound very poor.