Installing BMW Alpine Stereo upgrade (BMW 3 series E90)

I'm not an audiophile, but standard BMW stereo system sounds pretty bad to my ears. There are obviously very fancy (and expensive) audio system upgrades possible, but I decided to go with an official BMW Alpine kit. Mainly for two reasons: 1 - sound quality is considerably better than standard stereo (I'm not really interested in window shattering bass, etc). 2 - everything is plug and play and reversible - no modifications, splicing, wire cutting.
Car:
- BMW 320 (E90) Sedan
- Manufactured October 2011
- BMW Professional Stereo
Standard BMW Professional Stereo consists of:
- BMW Professional Head Unit
- 2 x front door speakers - 4'' (10cm), 4OHM
- 2 x under-seat subwoofers - 4OHM, not sure about size
- 2 x rear deck speakers - 4'' (10cm), 4OHM
Required parts:
- BMW Alpine Upgrade Kit (BMW part: 65412163268) - £360
- Door surround trim left (BMW part: 51337171201) - £25
- Door surround trim right (BMW part: 51337171202) - £25
- After the upgrade is completed, the Head Unit may require re-coding to "flatten" the built-in equalizer. BMW charges around £130 for this. In my experience BMW dealers were quite confused about this and gave me conflicting information whether the re-coding was required and why... At the end they did the "re-coding" for around £90, but I'm not convinced that it was required (or made any difference).
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