Not having Airmatic and ABC confused are you? ABC is the really expensive one. But it's optional on non-AMG and it wasn't a popular option.
As for the HK amp, I've had the problem. In my opinion it's to do with water damage - rain water runs off to the side of the bootlid, onto the strut arms, into the hole, onto the top of the amp, when you open the boot.
The amp is around £500-800 to buy second hand, around £1500 to buy new. No programming needed, direct replacement.