Guglielmo,
You are getting good advice from Richard Hess over on one of the Studer lists.
I would add a couple of things:
First, figure out the best way to plug in your power cord. Since the preamp has a two prong plug it can be plugged in either way, and one way will give lower leakage current than the other. Connect a meter between the chassis of the preamp and the ground post on the back of the 810, set it for AC volts, and try both orientations of the plug. The one that gives the lowest reading is the one to use.
Now hook a wire between the chassis of your McIntosh and the ground post on the back of your 810. Do this BEFORE you make any signal connections. Make some shorting plugs for the inputs that you will be using on the preamp--that's just RCA plugs with the two terminals shorted together. With the shorting plugs installed, the ground wire connected and everything powered on your system should be noise-free. If not, you have some problem in the preamp that should be fixed first.
But if it's quiet at this step then remove the shorting plugs and connect the 810 to the preamp using cables made according to Richard's advice: pin two to RCA center pin, pin three to RCA shell, and start with pin one connected to the shield of your cable but not connected to anything on the RCA end. If it's quiet, you're done.
If it's almost quiet but not quite, then try connecting the shield to the RCA shell.