Pull out the rectifier tube (or solid state gizmo). You can pull out the rest of the tubes if you like; it doesn't really matter. Power up the amplifier. Keep your meter probes away from the rectifier socket. You can check the bias voltage at F and G, which should be accessible from the top side of the Poseidon board. Turn the big bias pot all the way counterclockwise and the bias voltage should go to roughly -67 volts DC. I use an alligator clip lead and keep the black meter probe attached to the chassis while checking this. You can also adjust the small blue multi-turn trimpot on the Poseidon board to get the voltages at F and G roughly equal to each other. This is the best place to start your biasing. While you're here, set your meter to AC and check the voltage between D and E. It should be roughly 6.3 VAC.
Switch off the amp, let the tubes cool (if you left them in), and now you can install all the tubes. Proceed with the biasing instructions as described on page 10 of the poseidon-tz.pdf manual.