I can hear hum when using a dynamic mic with the TwinTrak Pro. It gets louder when I engage the 'air' switch. Is something wrong?

Unfortunately this is inevitable - dynamic mics can be good at inducing hum and the inductor circuit activated by the 'air' switch will only make it more audible. Setting the impedance to maximum will minimize the level of the hum but it doesn't get any better that this, simply because of the way dynamic mics pick up interference. The only other solution is to use a condenser mic.