what about just upgrading the speakers? I believe there was a vendor who posted an E70 speaker upgrade kit that would replace all pre-installed speakers, including the subs... for about $650 bucks!

if its acoustics and speaker brand/design maybe it pays to just swap them out...

i can't remember where the link is....but it may be the ticket we're looking for..

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2012 X5 50i M-Sport
2014 X3 35i M-Sport