Andrew, I wouldn't worry about what anyone is thinking - you have absolutely every right o demand perfection from audio system.
I would have the sub replaced by Merc, then get yourself to the very best 3rd party audio installation shop you can and have a pro opinion. Have them identify the route cause and write a letter confirming it.
I'd then ask them about remedial action and present Merc uk with the bill for doing so...
Gone but not forgotten:
325i M Sport (E92), Black Sapphire, Coral Red Dakota, 19" 225's, Seat Heating, 6FL and de-badge
Replaced by 6.2 litres of AMG powered C class.