Metasyntactic

Hello All, One of my speakers has popped after an accident (not my fault!) and id like to replace it. However, speaking to my local BMW, they don’t stock replacement speakers let alone upgrades for my model. Back in TT days, I upgraded the sound system to the Bose Opera –something or other. It was amazing! I'm looking to do the same with this one. I'm not technical in the slightest, so don’t know what you’d refer to the speakers behind the seats as (the two big main ones). But id like them to be something meaty/tasty that can carry the base. Not sure if ill need/want to upgrade the door card speakers – but can anyone give any recommendation for; a) The speaker that I should install (style, size, brand, spec, price) b) What I should be looking to pay a local specialist to install the two behind-the-seat speakers c) If theres a genuine/recommended BMW upgrade/brand style speaker. d) [Optional] a recommended fitter/installer/supplier in the Northamptonshire area. Many thanks in advance everyone!