Thinking about installing two 10" or 12" subwoofers in 2006 750 Li. Anyone do this? Any suggestions?

My 2 cents: Bavarian Autosound make a custom box that ports through the rear decklid for my E39 I have been looking at for a while. Not sure what they have for your ride, but there should be a shop that can custom build you something (+ you can hear the speakers live before you buy) Sound deadening on the rear shade might prove tricky, but lots of info on the forums about this. Depends how much you like having space in your trunk vs. how much sound vs. how much you are willing to spend. In my experience JL audio makes great subwoofers. In the end depends on music taste and how much sound you want/air you need to move to get the desired result. I have heard 2 10"s that made more & lower freq. sound than 2 12"s, so more space and sound can be reached with more $$.

you could also try a variation on the current Alpine factory upgrade and inline a new amp for the existing under seat subs, despite there size they can still be made to output a decent sound in both depth and volume.