Not a lot of people know about using the duct seal..that's some good stuff.
Deadening won't necessarily help the midbass, but it's usually used in conjunction with sealing because as the inside of the door acts more like an enclosed space it will have a tendency to rattle or resonate. But...because it sounds like you mounted the speaker well I'm wondering if it's another issue. Does it sound better if you adjust the balance all the way left or right so only one speaker is playing? Do you have any other crossovers besides the one on the headunit turned on? any EQ settings that may have the bass turned down?