FWIW if I was going to replace the woofesr it would not be with 'drop in' replacements. Looking under my seats it appears that you could use a standard driver installed facing down, leaving the vented covers off. The frame/magnet would stick up above floor level, but it looks like there's room enough so that it wouldn't interfere with the seat. That opens up the possibility of using much less expensive but very high quality drivers, for instance:
https://www.parts-express.com/dayton...-coil--295-508
That driver mounting scheme would increase the effective volume of the enclosure for better results. It's not something I'm interested in myself, but it's a viable option to investigate.