I would give it a try with RTV or maybe hot glue. You are unlikely to make things worse.
If you replace the woofer, try to get the exact one, especially if it's a ported box because the box is "tuned" to work with the driver. And assuming it's an active sub, the amplifier might be EQ'd to optimize performance of that woofer in that box, even if it's a sealed design.
There are places that specialize in speaker "reconing. They can repair almost anything and it certainly won't cost $700.
Or you can by a different woofer and use the Theile-Small parameters with speaker design software to check the performance in the box, and maybe determine some port modifications, or the software will tell you if a ported or sealed design is better with a particular driver, etc.