Nice looking rig. Are you using the Marantz as a pre amp?
There is a guy in the Akron/Canton area, whose name escapes me, who is supposed to be a certified Mac tech. I have used a guy outside of Harrisburg for vintage R2R tape deck work. He knows Mac as well. About a 3 1/2 hour drive. Terry DeWick is another well regarded Mac tech, in Knoxville TN; waiting time is typically three months. If all you hear is the hiss you have always heard, I am not sure you need service, at least on the amp.
I don't see a CD player in your rack. If you have one and it has a volume control, try connecting it directly to the power amp. I suspect you will hear an improvement vs going through the vintage Marantz