I just bought my first BMW, which is a 2011 X5M with premium sound (16 speakers, 600W) and it sounds ok, but I am not sure I get the "premium" point as with 600W there is no deep bass, there is base, but it's not deep. System is loud, but bass is not deep. My computer speakers (Logitech 4.1) have a deeper base. I have never had an aftermarket sub in a car before, but looking to have one or two added (do not expect to swap any of the existing speakers bur rather to add). The goal is to keep the trunk stock/empty and hide sub in the spare wheel space as I have it free. I was kind of sure on the JL Stealthbox, but read online that it rattles the lid that covers the trunk area. Also, I would rather use the wheel well and keep the close storage compartment available as it's more useable. I would not want to modify the lid in the trunk with wholes/openings/meshes installed and would prefer to keep that lid as is. If I am looking to spend ~500-1500 just for sub(s) with enclosures (and then amp with JL RD500/1 in addition to that) what are my best options? I am thinking of the way to have JL's W7 either one 10" or 2x 8" in there in a custom or prefab box but not sure it will fit. I have seen Audioson flat box with 10 for 400, but not sure how it compares to JL as JL seems to be the best.
My targets are:
1. good base that adds to music, not just a boomy disaster
2. empty trunk and the lid with no wholes
3. removable/transferable to another vehicle sub
4. preference not to change/upgrade any other elements of the system at this point so the added sub should sound good with the stock premium system

Can JL W7 be hidden underneath? 8 or 10? 8x2? Audioson? Anyone has had good results on 1-2k budget by adding to premium system?