Upgrading from HK to BW on s60 2019

[Cerebro]

First of all,  I don't know for sure what is the minimum impedance of B&W (AUD).  B&W speakers might be 8 ohm but that does not mean the amplifier is minimum is 8 ohms. 
Ask your local dealer if they can provide you the data sheet or the amplifier specs. 

But in general.  You should not connect a speaker lower than the amplifiers minimum impedance but you can connect higher.  Meaning, you should not connect 4 ohm speaker to a amplifier rated minimum 6 or 8.  Speakers will overload the amplifier and on high volume it will burn the output channel(s). 
If the minimum is 4 ohms like HK amplifier, you can connect 6,8,16 ohm speakers. no problem 

So, if we assume that B&W (AUD) is minimum 6 or 8 ohms and you connect 4 ohm speakers, it will eventually overload the amplifier and burn it.  
If you really want B&W AUD, my advise use B&W speakers or find speakers with 8 ohm. 
Now, for the MID + HIGH is different story. They are in parallel so the total impedance calculation is not 4 + 4 =8 , it is 1/4 + 1/4 = 2 ohms total.  So, you must have higher ohm for tweeter and mid to reach 8 ohm in total.  Some tweeters and MID come with filters. You need to connect them in parallel and measure the total ohm. 

DO NOT LISTEN to id*t sellers. If the amplifier is 8 ohm and you connect  4 ohm speakers,  it will burn your amplifier little more than high way volume and sometimes it even burns less than half volume depending on the pre-amp (equalizer) settings and how the song is recorded.

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I just can't thank you enough for the best answer ever. I have been dealing with this question for two weeks now and my head was about to explode hehe