We did a thorough upgrade of the sound system in this BMW E30 325i. The customer already had a JVC deck installed but found the factory speakers lacking. BMW's factory speakers were 5.25" in all four corners and didn't offer quite the clarity or bass response that our customer was looking for. These are great driving cars with amazing steering feel but the audio quality from the factory amp and speakers is typical of the 80's.
We did an amp bypass and installed an Alpine 5-channel amplifier to power the 4 speakers and a 12" Kenwood Performance Series subwoofer. These 5-channel amps are great in terms of packaging as it's a 2-in-1 system making for simpler wiring and space-saving. The Alpine 5-channel amp can provide up to 300W to a 2-Ohm subwoofer which is plenty for a small sedan like this BMW.
For the speakers, we installed the Kicker KS series speaker. Although the factory front speaker in the footwell was a 5.25", we upgraded it to a 6.5" coaxial for even better sound. On the rear deck, the Kicker 5.25" was a perfect fit in the factory speaker pod. With the upgraded speakers, amplifier, and subwoofer, the sound quality and volume across the entire frequency range is greatly improved from factory.