I have the HK system and I will say that of all of the BMW's that I have owned and all that have had the HK system, the one in the M40i is the worst, it truly sucks.
I talked to Bavsound about replacing the speakers and they have a kit that they can offer, but it is mainly only 4" speakers, which doesn't sound like much of an upgrade in the stage, so I just deal with it until someone comes up with a much better solution.
I sent a bunch of pictures to Bavsounds and they, as I, were surprised that there are absolutely no subwoofers in the HK system, so this is pretty much what they offer for an upgraded sound stage.
https://www.bavsound.com/Product/bsd...one-bmw-e89-hk