My car came with an amp and upgraded speakers. There is a subwoofer box that is unsecured in the cargo area, that takes up too much space.

Mtld83 on reddit is convincing me to go for a JBL Bass Hub. YouTube video showing Bass Hub innards, showing a downward firing configuration. More reddit comments. 2addicts.com thread.

The volume in the spare tire can be slightly larger than under a single seat. Also, it's easier to clean under the seat without a box. Also, no impact to foot space. Tools that go in the spare tire volume can be placed elsewhere under the cargo space floor. However, my spare rim is 4“ wide and is dished outwards so the inside height is maybe 2”.

Add pillow stuffing to subwoofer enclosure to make bass sound fuller (without the air-knocking sound as pressure waves bounce off walls of enclosure). See mazda247.com forum.