Cheers for the update on your sub install. I just had a look at your rear tweeter installation and it's something I've thought of doing. Doesn't sound too pricey either, so potentially worth a punt.
I've got the car for another year so I'll still be doing a few upgrades yet!
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Current car: 2017 BMW G30 520d M-Sport, BMW F46 218d Grand Tourer
Previous cars: 1989 BMW E30 316i, 1998 BMW E36 323i Coupe M Sport, 2009 BMW E87 116d, 2013 BMW F31 320d M-Sport