@2000m2 - If your current HK has pre-outs, then the VC on the HK should control the volume when you have a power amp connected to the pre-outs. Seems to me the HK either does not have pre-outs, or you are connected to the wrong outputs. Are you sure you are not connected to tape monitor outputs?
What model HK do you have? Can you confirm the labeling on the outputs you are using?
What model HK do you have? Can you confirm the labeling on the outputs you are using?

