If you are talking about the high voltage secondary center tap, yes it must be grounded. That's the main circuit ground (common).Thank you. Yeah, hum is part of the current problem. It came to me with the IEC 3-prong, and I have no problem putting it back to the original circuit if that's the overall consensus.
Replacing all caps now. The hum follows the volume control a bit. I can hear both 60 and 120hz. The 120hz goes away when the amp is run at 110 volts.
Another question: Should the transformer center tap be grounded to the chassis at all?