I am running a Behringer mixer into an interface strait into pro tools. Its a pretty old mixer, the model number is Behringer Eurorack MX 2642A. When I had a friend come look at it, he said that i was getting alot of unwanted feedback. This might be because i might not have pre-amps. does anyone know if this mixer has pre-amps? and would it be cheaper to buy a new mixer than buy a pre-amp for each channel?