MIDRANGE (AND/OR MIDBASS) SPEAKER INSTALLATION
How the midrange speakers are physically mounted will have a tremendous impact on how the system will sound. The midrange speakers will handle everything from around 60 to 90Hz on the low end up to around 3,000Hz on the high end, depending on the manufacturer's recommended crossover points. This frequency range represents the most critical 5+ octaves of the 10-octave audible frequency range. Subwoofers operate in only the lowest two octaves or so, and the tweeters in the highest three octaves. And, most of the information in music, the majority of the vocal range, and in fact the majority of everything we listen to, is contained in the frequency range reproduced by the midrange speakers.
So, if the midrange speakers don't sound good, then it's very likely that the entire system will not sound good either. And ensuring that they sound good is in large part determined by the quality of their installation. While a midrange or midbass speaker isn't as large as a subwoofer, it still must be very solidly mounted. The moving mass of the speaker cone, especially at high output levels, will try to move the entire speaker, and the surface it's mounted to. If it's not mounted properly on a solid, non-resonant surface, it may cause parts of the mounting surface to rattle, buzz and cause unwanted noise and distortion.
If the OEM speaker locations are used, the mounting structure will likely need to be strengthened and stiffened. The very thin metal and plastic (which is becoming more ubiquitous) that the OEMs use may work decently for low-powered, less-than-great-sounding OE speakers. But we are installing high-quality gear, and want it to sound good, so the mounting surface needs to be improved.
Depending on the vehicle, this may entail simply using good hardware to install the speaker. Or it would mean adding some additional structural material, completely reconstructing the panel, or even building a dedicated enclosure.
Also, if there are holes in the mounting panel, like the access holes in the door panels to allow service of the window and lock mechanisms, they must be sealed off so that the back wave from the speaker doesn't interfere and cancel out the front wave that we want to project into the listening space. Small holes (less than an inch in diameter or so) can be sealed easily using the adhesive-type panel damping materials. Larger holes will need a more solid structural material to be utilized.
Material selection for these purposes can range from ABS plastic, to fiberglass, to aluminum, or even steel. For door installations, wood or MDF usually isn't a good choice because when it gets wet from rain, it can warp, swell, or break down and fall apart. And, backstrap or small thin brackets won't do much good either, as they cannot provide any real strength for the panel, or help seal large holes.
The sound quality improvement will be directly related to the quality and effort put into the structure. Build it right and it can sound great.