HiFi (Option 676) is the easiest and cheapest architecture to upgrade.
WARNING: There are known manufacturing defects for OEM Head Units produced from 2009-2014, BMW have published a service bulletin for the issue. It's most prevalent on HiFi cars. See this link for details
The signal from the head unit is
low level, full-ranged, flat / unprocessed and uses balanced differential outputs. Thus, it will result in the
highest-quality analog upgrade possible (There are some digital options coming out, see Top HiFi section below).
Note: your amp must be capable of accepting balanced differential outputs.
CTuna's HiFi Install DIY w MusicarNW Kit
My E91 VP Kit install.pdf
Budget Hifi Upgrade
Midrange HiFi Upgrade
Nice HiFi Upgrade
Very Nice M3 HiFi upgrade
Technic's
"Ultimate" HiFi Upgrade