As a side note to a dampening/midbass discussion thread, I want to add that the real reason why the midbass improves is that door speakers are usually an "Infinite Baffle" type of enclosure loading for midbass woofers located in this location. Infinite Baffle is sometimes called "Free Air" as well. If there is not nearly perfect isolation between the front and back of the speaker, then the sound wave that comes off the back of the driver leaks into the wave being produced by the front of the driver and this causes cancellations that lead to greatly reduced output and an overall "muddy" kind of sound. Also, if you use acoustic deadener material to reduce vibrations anywhere on vehicle panels, then the result is that less vibrational energy is "stolen" away from the system and this can increase the SPL produced by the system. Increasing the output even 1dB due to this can be noticeable. Most auto door speakers are purpose-designed to have a very large air space behind them vs. the size of the cone. That's why those "acoustic baffle" cups often cause all the low frequencies to disappear from a door speaker when they are used with them! A rule of thumb for good infinite baffle driver will have 4 to 10 times the driver "Vas" of space behind it for optimum performance. Another "rule of thumb" is that a Qts higher than 0.7 indicates a good chance a driver will perform well in Infinite Baffle/Free Air situations. But, all the driver parameters must be considered together, not just Qts. (Sorry, I don't want to get off track here, just giving more info for the "curious" types to do some Google search on their own for this subject...)
I don't want to get into a discussion about the physics of why/what/how some things are done to change speaker enclosure loading, as many of them are highly opinionated by those who write them, and situational as well. For a "TL;DR" summation, suffice it to say that when regarding door speakers, deadening/isolation is good, while restricting the space behind a woofer can be bad depending on it's characteristics. So, be careful how you treat the space BEHIND the cone while making sure that you do a good job of using deadening material on the inner door surface to provide front-to-back isolation.
Hope this helps! //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif