While you have the sub out, it's easy to upgrade. Open the casing and lightly fill the inside with poly filling. You can buy a bag of speaker poly filling, or use the poly stuffing out of a stuffed toy or pillow. I bought a small pillow at Goodwill; it worked great.
The most important upgrade is to cover the exterior surface with a vibration damping material. Local car audio shops will have several choices. Here are some examples:
https://www.parts-express.com/cat/sound-damping/409
Sound out of a speaker needs to come only from the movement of the speaker cone. The speaker cone pushes waves in the air to our ears. If the speaker cabinet is also vibrating different sound waves come through the air muddying the sound.