I did same exact thing with my X3.. I would suggest upgrading your amp first, which is in the trunk behind driver (probably passenger side in your case) side trim panel. Stock midbasses sound pretty good with more power, I have upgraded to SWS that bolte in perfectly with no spacers required in X3, and the sound is only slightly better than stock. My guess would be due to X3 being sealed enclosure. I had these subs in my e90 and the sound was much deeper and pronounced.
BSW speakers sound much happier with a little more punch. I upgraded to 6 channel JLAudio XD600/6 amp and used Technics wiring harness (no wire cutting).
2014 F25 X3 M-Sport Carbon Black, Technology/Premium/Winter/Driver Assistance
2011 e92 335is DCT - Cobb, ETS IC, AFE DCI, BMW PS kit, M3 control arms, various cosmetic improvements RETIRED!
2007 X3 RETIRED!
2006 e90 330i 132k miles RETIRED!