To answer my own question.... I rearranged the shielded audio cable so that the input line remained shielded right up to the connection and ran a ground up to the shield right at the input connection. MUCH improvement. With full volume and full gain at the amp there is still a faint hint of a hum but this is such an improvement that I am satisfied now. It's not dead silent but close enough. It sounds great, drives a 12 inch speaker with no problem and is plenty loud. I think it's time to find the right speaker now and package it all up.