Well folks I had been looking for the wiring (plug) info for this radio for quite some time and finally found what I was looking for. So for those who have also wanted to use a different mic here's what I found. This info is for the the CRT SS-9900 which is simply another rebadged at-6666. Hopefully this helps someone else. P.s. stay away from the center pin as it is 8 volts and you don't want magic smoke!
I found that out the hard way...lots of smoke from these little
After much debate about that stock mic on other threads and forums it was determined that it is a high impedance electret condenser mic. That would make sense to me because so many rigs are going that route nowadays, plus it would also explain why substituting a low impedance dynamic would sound inferior. Power dynamic mics will make up for the impedance difference, but IMO that’s not the route I would pursue. If noise cancelling is an issue, I would first try other more robust electret mics and / or mod them to decrease their sensitivity to background noise. Electret mics usually have a wider audio response by nature, therefore they sound more natural. Unfortunately it's that same wide audio frequency range that will pick up all the highs and low sounds in the background making it necessary to cut off some of that range in a noisy environment. A tailored mic covering only your vocal range is desirable in a noise canceling situation, otherwise I let the HiFi rule the day.
This video discusses the same thing as you have said about mismatched impedence.