I have a chance to get a TenTec 215 mic for my Omni-D that will require cash, or a free Shure mic (unsure of model) that is a 500 ohm impedance. The rig docs just say high impedance, nothing specific. I am unsure of what high impedance actually is. Will a 500 ohm mic work with my old Omni-D? Glenn, WE9K