Could be a short at the speaker jacks. Turn amp off. Unplug the speaker and measure the resistance right at the speaker, it should be about 6 ohms. Plug the speaker into the amp and measure the resistance at the speaker again, it should be pretty close to a dead short. Unsolder the green wire that goes to the output transformer and measure the resistance, it should now measure 6 ohms again. If it still measures a short there is some kind of short at the jacks.
What are the voltages at V4 pins 6 and 8?
-26V on the grids of the 6V6s seems a little high. Do you have any way to measure the bias current?