In both the amp and preamp, the high voltage is always on when the back panel power switch is turned on. The tube filaments are turned on when the unit is taken out of standby. One advantage to this is the high voltage regulators are always on and warmed up.
While in standby, all of the high voltage and output stage regulators are on and the whole solid stage circuit is active, but with reduced current in all of the active devices. This way, the circuit will take less time to get up to full sonic quality when set into operate mode as opposed to starting from a total cold start and putting into operate mode right away.
Regarding the warmup - The time delay allows the tube heaters to come up fully and the plate voltages to stabilize. The tube heater voltage is applied full on. The delay also gives the coupling capacitors that feed the solid state output stages to fully charge up as their outputs are shorted to ground during the time delay. Then, the coupling capacitor outputs are unshorted. This is true of both the preamp and the power amps.