80 hz is very achievable with many aftermarket speakers.
Let's look at the situation though. Stock speakers are generally paper cones, and very light at that. Resonant frequency (Fs) is pretty high because of that, but due to the very light materials the driver can play well under the Fs. What does that translate to, really crappy midbass. Distortion is extreme, and overall the driver sounds like crap.
Now, on to aftermarket. There are several ways to approach an aftermarket speakers. Sparkling highs, or deep lows. Most people respond to sparkly before they respond to bassy, so companies like Infinity build their speakers to suit. However, in order to build a speaker that runs relatively distortion free so your high end doesn't get muddled up, while also being able to be ran in a car door, you need to do several things. In order to keep a driver controlled in a car door, you need to keep it from reaching it's Fs is an open/infinite baffle environment. The easiest way to do this is to raise the Qts of the driver. The Qts is a combination of Qms (mechanical Q) and Qes (electrical Q). The easiest way to raise the Qts is to raise the Qms. Mms is basically the mechanical function of the driver, where mass of the cone, coil etc are figured in and is a factor in determining Qms.
So, with that said, how do we lower the response of a driver with a lower Qts, while still maintaining low distortion levels? We control the environment of the driver. There's a reason that home audio speakers are mounted in boxes. By deadening and sealing your doors, you effectively create an enclosure type environment, though it's still a crappy environment by most standards. By that you allow a driver with low distortion, and lower Qts to operate efficiently.
Clear as mud???
