There was a slavish notion back in the old days about 600 ohms , if you need to be able to energise a twin unscreened pair over hundreds or thousands of yards maybe its appropriate , but over your standard 10meters or 33 feet of tube mic cable, a cathode follower can happily deliver a single ended signal with low enough impedence that the cable capacitance is irrelevant to several hundred khz , the B&K cathode follower mic is a great example .