A great cheap speaker - Pioneer TS-A6886R 6" x 8"
My pasenger door speaker was blown, so I threw in a replacement. If you decide to upgrade your stock door speakers, and you have 6"x8" speakers, I think you would have a hard time finding better speakers for the money than these. They are only $40 and are a much better speaker than what Miatas came with. The bass is surprisingly decent considering the less than ideal sound environment. I have installed the 6"x9" and 6 1/2" versions in my girl's ride as well.
I have a dedicated amp and expensive 6 1/2" Infinity components in my other car. Compared against them, the coaxial Pioneer's tweeter isn't quite as good, but the woofer is better. It is truly amazing how cheap car audio has become over the past 10 years. Crutchfield and Amazon will say that the speaker will not fit in a Miata, but there is no problem fitting them behind a NB Miata grill. They don't contact or rattle.
Normally, you should never run additional tweeters with a coaxial type speaker. It screws up the imaging of the sound stage and can cause interference when the crossover point is different. I only drive my car with the top down, so I A-B tested with the additional door tweeters hooked up or left disconnected. The door tweeters definitely helped in raising the sound away from the floor. I decided to sacrifice some audio quality for the higher up sound since I am going to have high wind sound levels messing things up anyway. The speakers are plenty efficient and get ear damaging loud off a stock head unit.