I did last weekend a "humble"modification to my X3 06 changing the original 4 door speakers for Hertz Dieci DCX100. I noticed a huge increase of quality sound just for 90€. The Hertz has the perfect mounting deep (42mm) for X3.
My next project will be change the beneah seats speakers for two Hertz energy HV200 almost without modifing the original box.

I'dont know if thi is a valid solution for you... but it's very efficient Cash/improvement.