Randy, what removing that screw does is to cut the connection between the grounded sub amp chassis and the shield in the sub cable. Normally, having them connected can slightly reduce the noise level by having the shield also using the chassis ground to drain off noise. If there's a ground loop present however, with them connected it also feeds into the cable and then to the receiver. The difference in the electrical voltage potential between two different electrical circuits can cause a hum current to flow through the amplifier and be audible in the speakers. Removing the screw(which still leaves the sub amp chassis grounded)might slightly raise the ultimate noise floor, but doesn't feed the ground loop hum into the sub cable.
As to positioning the subs, having the two EP350s up front supporting the VP180(possibly flipped upside down to project more upward?)and just left and right of center is very little different from having one sub at the midpoint of the front wall from the standpoint of the room mode minimization placement suggested by Dr. Toole and others. Having the EP600 as close as possible to the midpoint of the back wall would therefore form the midpoint of front and back wall arrangement and should work quite well.
