I guess I can't joke too much about your car having a terrible stereo - although it's a frequent topic and you should have found it without posting - but your SA and your service department should have explained this to you. I'm more critical of them, since you did what you should do, go to the BMW service department.
A US-model Stereo system can be easily identified by looking at the top of the
door panel, inboard of the side mirrors. If there are no tweeter grilles present
there, the vehicle has Stereo audio.
The Stereo system consists of:
- A head unit coded to Stereo mode, providing speaker-level analog signals
- Front door 4” cone speakers using the standard BMW 3-bolt frame
- Rear 4” cone speakers (rear deck, doors, or sides, depending on body)
- Underseat 6” cone speakers with 8” frames, inverted-mounted in 8”
These speakers are powered by a 4-channel amplifier IC inside the head unit.
The iDrive head unit has slightly more power than the Professional CD player,
but both are limited in performance and can be thought of as having similar
output as an aftermarket CD player.
To compensate for the lack of tweeters and of larger 8” subwoofers, the head
unit front speaker signal is heavily boosted in the bass and treble. This
equalization curve is fixed at all volume settings.
The front 4” door speakers and the underseat 6” speakers are both connected to
the front channels of the head unit output in parallel. The 4” rear speakers are
connected to the rear output channels of the head unit, and that signal is
crossover-filtered within the head unit with a roughly 150 Hz high-pass filter.