Are you interested in adding a subwoofer or replacing the under-seat woofers or both?
Musicar NW out of Portland Oregon could provide options in addition to the ones you mentioned.
Replacing the under-seat woofers wouldn’t add much in my opinion. They are in a terribly location with installation limitations and compromises. Adding an additional true subwoofer and amplifier while also adding an additional amplifier to power the under-seat woofers while extending their mid-bass range and filtering off the low bass notes now handled by the subwoofer would add a performance boost. Replacing the under-seat woofers would add some performance gain but not necessarily enough to justify it. Adding the additional quality amp to power them would provide the most gain.
If you have run flats, the subwoofer and amps can be placed in the storage compartment a spare tire would otherwise be. If you can’t place a subwoofer in the storage space, a corner loaded custom enclosure is an option.
I initially did a bass-upgrade on my 335i. It added a lot but I ended up upgrading the entire system as step 2. For my X6 I just went straight to a full system upgrade.
For most upgrades you would need a MOST 25 interface, assuming your car uses it, I use the Helix Smart Digital Fiber-Optic Interface for MOST25 Platforms (HLX SMDI). With a full system upgrade I also use a high end Helix DSP.
This is what I had done to my X6, well beyond what you are considering, but the subwoofer part could be done without all the rest.
https://f15.bimmerpost.com/forums/sh....php?t=1681701