First thing to look at is the Voltages at each tube socket terminal. This tells us things are hooked up and operating as they should. Errant Voltages can also point to wrong value resistors. Also measure the Voltage at the plus side of the large filter capacitors and post the readings.
Another common problem is a short at the speaker jack. Because the output transformer winding is a low resistance, testing with a meter is tricky. The easiest thing to do is connect a meter across the speaker's terminals with the amp connected, but turned off. You should measure a low resistance, less than 1 Ohm. Now unsolder one of the output transformer wires at the speaker jack. The resistance reading at the speaker should go up to about 75% of the rated speaker impedance. If it doesn't, you have a short at the speaker jack.