Your loop jumper and speaker wires are in the correct pin position so no prob there. The speaker thump says all is OK, but the best way to check it is to unhook it and put it across another speaker from some other equip that is known working.
The audio power amplifier in a 790 has two inputs and two outputs (guess the chip was made for a stereo). This is for speaker A/B operation. The B speaker output goes directly to the secondary speaker without jumpers or whatnot. Speaker A output only has one jumper, the loop wire on the back of the chassis connector. Speaker A can be hooked to the chassis connector, or the control head courtesy of the loop wire. Speaker B can only be hooked to the connector on the radio chassis.
Switching for this dual speaker setup is done by the radio (ahead of the audio PA) which sends audio to either input A or input B for whichever speaker you want to use.
Now before it gets too deep, remove the speaker from the head and plug it in to the chassis connector pins 7 and 8, the speaker B output, and see if you get anything. To remove the pins if you don't have the correct removal tool, you'll need to use something small, sharp and pointy to press in the little captive "finger" of the pin. It'll slide right out.
Let us know how this works...