[Alaska]

I know, “use your ears”, but Im curious some specific mics you would use 300 with or 1200 with.  What is the general usage rule of thumb for choosing these?  I have all manner of mics, and 1200 just seems to be more open sounding in my limited testing.  

[JPK]

300 ohms gives 6 dB more gain if the mic has low impedance, but loads the mic more, which can result in restricted bandwidth if the mic has high impedance.

[Alaska]

What is considered “high impedance”.  Any specific mics you would use 300 on?

[JPK]

The mic impedance should not be higher than the preamp input impedance divided by 10.
Pre on 1200: use mics with less than 120 Ohms (most mics)
Pre on 300: use mics with less than 30 Ohms (ribbon mics generally)