Before you do that let's go back to the RCA connect. You said with just speakers, amp and pre-pro you get buzzing. I'm assuming the Monster power converter is also hooked up?

What I'm thinking is there might be a ground offset between the pre-pro and the amp. If the offset gets big enough then even balanced connects are not going to help. It also makes sense to me that adding more cables to the pre-pro could make the ground offset worse. Can you ground strap the pre-pro to the amp somehow? Using the cheater plug with the RCA setup would also be interesting as that should float the amp earth ground.

However if you really want to understand what is going on, let's put the volt meter to work. We'll (i.e. you'll) be checking continuity so unplug the Monster from the wall.

1. Unplug everything from the Monster. Put the voltmeter on Ohms and check for continuity between the third prong of the input plug and the third prong of the output plugs. If the pictures I found of the internals of this unit are correct you should read zero ohms.

2. Now check the other two output prongs back to the ground prong of the output (or third prong of the input power if they are tied together). I can't tell from the photo if the neutral wire is tied to the chassis ground of the Monster. This will tell us.

2a. It looks like the outputs of the monster may be isolated from each other. Measure between two output power ports and seee if there is any continuity.

3. Moving to the pre-pro. Unplug everything but the power cord. Check for continuity between each of the two input power pins and the ground on the RCA jacks that goes to the amp (just to be clear that's the shell of the RCA jack NOT the center conductor). Check all RCA jacks to each of the power input pins. All these measurements should result in an open circuit. (Note: This is 14 measurements for a 7-channel system)

4. Now the amp. Do the same measurments as you did for the pre-pro but add the third prong of the power input for a total of 21 measurments. I would expect the RCA shells to be tied (0 Ohms) to the third prong of the power connector (earth ground). I don't know how Outlaw handles the neutral input on their equipment. This test will test us. If you see zero ohms between the third prong and one of the other prongs then the nuetral is tied to earth ground in the chassis.

5. With your newfound understanding of your system you should be able to draw a diagram of where the power and grounds are going for these three units. If not, let me know what you measure. I already have a sketch of what I think is going on but I don't know the internal connects for the units, so no smoking guns yet.

Whew, that was a lot. If you're still reading this, congrats. You might just be an electrical engineer