Why does my midbass sound worse after mounting it on an adapter ring?

That thinner low end is almost certainly related to the air loading behind the speaker cone, not a leak or the added depth of the MDF ring itself. When you move a driver into a custom mounting solution like this, the effective air space imediately behind the cone changes significantly, and if that space isn't properly managed, the driver struggles to move the air it's designed for, leading to a loss of that characteristic midbass punch. What you need to do is consider the cavity behind the adapter ring. Even if the speaker cone itself is sealed tight to the ring, the air sitting in the rest of the door panel cavity acts as a buffer, and if that air is unrestricted or poorly managed, the back wave and the driver displacement are disrupted. You should be sealing the back surface of your wooden adapter ring directly to the door housing structure. This means filling every single gap between the ring and the door frame interior with a good automotive sealant or high-quality acoustic caulk. The goal is to create a defined, consistent air enviroment immediately surrounding the magnet and cone. Beyond the seal, you need to consider resonance. The door panel itself, which is mostly thin plastic or treated cardboard, is going to vibrate heavily when the speaker moves. Even if you seal the ring perfectly, the panel is still acting as a massive resonant chamber. If you can cut holes into the door panel behind the speaker, or even use heavy sound damping material, you need to decouple the speaker assembly from the resonant energy of the door structure. This dampening helps the speaker translate the driver's movement into sound waves instead of just vibrating the door panel itself.

By sealing the ring to the door and damping the surrounding plastic, you effectively stabilize the environment the speaker is working in. This should restore the performance you had with the original stock setup because you are bringing the speaker into a controlled acoustic environment, rather than just placing it in a solid wooden box that is floating within a noisy cavity.

I'm not totally convinced sealing the back of the ring is the fix here. While airflow and resonance matter, sometimes the problem is the mounting rigidity itself. I've seen drivers lose punch because the stiff MDF adapter just vibrates against the cone. You might need to address the clamping pressure or use a softer mounting surface, not just focus on airtight seals around the housing edges.

I've ran into this before with other drivers, and it might not be the mounting or the seal. Sometimes the paper cone quality is just off, or the magnet has a low Xmax rating which means it can't move as much air as needed even if everything is perfect. Check your driver specs to see what displacement they're rated for. Also, have you tried swapping the speaker for a diferent, known good one? It'd save you the headache of wondering if your build is the problem.

I once had an issue where a custom mounting bracket felt too light. It made the driver seem disconnected and lacked that authority you expect from a midbass unit. For my setup, the difference wasn was definatly using 3/4 inch thick MDF for the ring instead of the thin material I first started with. It really anchored the speaker in place. A thin ring just vibrates with the speaker movement and introduces weird harmonics that steal some of the low end power you think you're hearing. Have you checked if the speaker has any specific mounting pattern suggestions? Sometimes even with perfect sealing, if the mounting hardware is loose or the ring isn't perfectly flat against the speaker flange, you just lose the low end performance.