From MetroAmp Wiki
I think it might be helpful in re-wording the connection of the shielded cable. Using the word "grounded" implies that the shielded cable should be connected to ground potential (pot casing), which in turn would ground the bias supply. Obviously this could result in damage to the amp and/or tubes - at the very least blow a fuse.
The term "shielding CONNECTED here" would probably eliminate any confusion.
I have modified the wiring diagram image with these advisories.
