I would suggest mounting it where mine is, but using a PDX-5 Alpine Amp. This will make returning your sound system to factory much easier should the need arise, less to remove and the amp I suggested will mount very easy without bloody knuckles or the Yosemite Sam cursing that comes with many jobs. I kept my bmw 8 inch subs for almost a year before changing them out and the added power made them sound so much better until I upgraded.
They are now down to $450.00 which is quite the price reduction. It is actually two amps in one.
4 channel for the doors. x 75 watts
1 channel for the subs x 300 watts.
Although this may seem like overkill it is inherently the same idea as our engines. It will be a very rare day that you hit 137 mph, but having all that power in check means never having to work your x3 hard to get up to highway speeds of 80.
Your amp will always deliver smooth sound without ever running hot. And heat equals distortion.
2013 S4 Loaded (and NO RFT's) Awesome is an understatement
2008 MT X3
RIP you were a faithful sidekick and will be missed.
2007 AT X3 RIPOS
2000 Jeep Cherokee
1997 Twin Turbo RX-7
1984 GTI Wolfsburg Edition Neuspeed and more