I am 99.9% sure COMAND doesn't have line outs for amps. Few, if any, manufacturer fit units do.
You will probably be best taking a high level input from the nearest speakers, or from the speaker wires coming out of the COMAND unit (some cars have a dumb unit in the front and the actual Nav processors and audio amplification in a metal box in the boot which is known as an 'Audio Gateway', 2007 W211 certainly did).
I had an infinite baffle set up in the rear deck on my E500 5.5 and the installer took the signal from the Audio Gateway in the boot.
If the car has Harman Kardon (or the it will have a sub in the boot/rear deck already and that's also a potential place to take the input from. If you have HK or an Audio Gateway, anything upstream of the amplifier will be a digital MOST BUS circuit.