Removing the OEM radio disables the gongs you hear when your car is running on "fuel reserve," don't have your seatbelt on, bulb out, etc.
You also lose OEM BMW assist.
To keep these features, you need to relocate the OEM radio to the trunk with the harness in one of the threads here in the audio section.
From there you can recode the HU from L7 to Hi-Fi so you get an analog signal. However, I think you need something to clean up the proscessing from the HU so you have a useable signal for your amps...I may be confusing this with L7.
Once you do the above, you can add amps, aftermarket HU, speakers, sub, etc. while retaining the OEM gongs, BMW assist, etc.
Search for "relocate OEM head unit" or the like to find the thread I am talking about.