Although the G series cars have a very similar audio lay out to the F series cars, BMW changed how the 'base' audio cars were wired for the better, with separate channels for the under seat woofers. All 'head unit' modules are also now in the boot meaning these are slightly easier to upgrade than the F series cars.

 

In the UK, BMW offered 3 audio levels in the 3 and 4 series platforms.

 

Base Audio - no amplifier, no tweeters, 4" in the front door, 6" under seat 'woofers', 4" in the rear door.

Hi-Fi - amplifier in the boot, tweeters in the front doors with non branded grills, 4" in the front door, a centre speaker, 8" under seat woofers, 4" in the rear door.

Harmon Kardon - more powerful amp, separate booster amp for the under seat woofers, tweeters with branded grills, 4" in the front door, a centre speaker with tweeter, 8" under seat woofers, 4" in the rear door with tweeter, surround speakers.

 

While the Harman Kardon in these cars isn't 'bad' for a factory system, there is still a huge amount to be gained dynamically and tonally in adding a DSP amp, replacing the cheap factory speakers and adding a dedicated sub to give real sub bass.