Replacement of the OEM amp requires a 6ch aftermarket amp with balanced-inputs.
$500 can give you an XD600/6 and the DLS R4 component set for the fronts (if this is an E90) at eBay.
-2015 F30 320i, Imperial/Oyster, 8AT: August 2014 ED
-2012 E92 M3 Jerez Black/Rust Brown, M-DCT: Gone
-2008 E90 M3 Melbourne Red/Bamboo, 6MT: Gone
-2007 MK5 GTI United Grey/Grey, DSG: Gone
-2004 E46 M3 Carbon Black/Cinnamon, 6MT: Gone