Replacing HK speakers
If you want to keep the factory nav+HU, here are the integration solutions to add an amp to the factory HU:
http://mobridge.us/compatibility/mercedes-benz/
This would be suitable if you want to buy an analog RCA amp:
http://mobridge.us/product/da2-analog-pre-amp-most/
or this one:
http://mobridge.us/product/da3-dsp-professional-most/
I have no experience with this company, just found them through searching. I used Connects2, a British company that manufactures integration interfaces. I have full SW control, rear camera integration, and MOST amp connection.
I have always disliked HK systems. The KRX3 sounded amazing in my previous E55.
Another solution is to buy the 3.5 pods for the A-pillars. The shop who installed these in my previous E55, told me they sound far better ON-axis so I might give that a try. Focal suggest placing them ideally high in the doors.




