Thanks for that^^. I always wondered which group of rich smart people decided that 600 Ohms was the middle of the road.

The Hi-Z/Lo-Z difference comes from the research by our friends from Bell Labs in the old-timey days. The folks at Bell discovered that 600 Ohms was the best impedance to run vocal frequency audio over the longest distance without signal loss. They were interested in getting a telephone to work over long distances. The low frequencies and the high frequency don’t work well at this impedance. Think the band-passed “telephone-voice” effect in alt-rock. After a while, anything that had impedance lower than 600 Ohms was considered Lo-Z or low impedance (like ribbon and dynamic mics especially) and everything above 600 Ohms was considered Hi-Z or high impedance (like electric guitars and line level equipment).