just got back from measuring, the resistance on the 2k ohm scale is (.02, .002, no reading) and (.019, .002, no reading) for each tab, going from left to right looking at it from the powerswitch side.
I think we need to clarify here the difference between the tab and the pins.
The tab and middle pin are internally connected. That's why you're supposed to electrically insulate the tab from the heat sinks.
On this amp, the drain/tab/middle pin is connected directly to the 48 V supply. If your heat sinks are grounded and the tab touches them, you're shorting your PSU. That would explain why nothing works.
And well, if you're getting 2 ohms between the middle pin and the copper plate, which I assume is ground, you've pretty much confirmed a short.