I have been there and I have done all sort of experiments.
I started with normal JBL amplifier (GTO series which is excellent).
After coding HU (balanced input), and connecting it to standard amp, there was noise due to differential input. I used standard LOC for this and that did not help at all.
I had to add LOC (which converts balanced differential to balance standard signals) to get rid of noise and to be able to use normal amp with differential BMW HU output.
Lately, I've moved to JL amp which accepts differntial input and now I don't require LOC.
HU---> JL Amp ---> Components and I must say, sound quality is way better than previous JBL and Kenwood amp which I tried earlier.