The L7 amp gets its sound info from a fiberoptic link which is also one of the
devices on the MOST BUS.
No sound without that link and only a BMW Head unit provides it.
When you change the Head Unit there is no more link.
Yes no sound.
And no other aftermarket manufacturer makes a head unit compatible with this type of fiber optic interface. (Well unless
they have just made one)
You could run the 4 amped channels off the head unit but it will
be a poor substitute for the l7 sound wise and it will need a harness to route the channels.