So, increasing the number of speakers will produce more sound, but getting cheap speakers will just generate more noise. The same with throwing a bunch of tweeters in the mix. A tweeter was really only designed to produce high notes, so you would just be overwhelmed anytime notes above 2,500 Hz were being played. And if you didn't wire them up with the proper crossovers, it would just destroy them in a matter of weeks.
If you want good fill in sound, then I would suggest getting a nice set 6x8 for the all of the doors and hooking up them up to an amplifier (Unless you want to invest and get components, they would do even better).
As I mentioned in another post, most radios claim to be "50x4" watts (Or somewhere in that range), but that is a
PEAK number. By, getting a set of speakers that is capable of handling 50 watts RMS and hooking them up to a decent AMP that produces the same power, they should generate a lot more clean sound, and the higher watts usually help produce better quality of sound.
One thing to always remember when it comes to car audio and SQ...
Quality > Quantity
There is a discussion a little lower regarding some Boston Acoustic speakers that are fairly resonable in price.
Check it!
