Biasing a tube is adjusting the amount of current that flows between the anode and cathode of a tube. Exceeding tube manufacturer specifications at the least will result in shorter tube life possibly stressing the tube to failure. Like many amps, from the factory the EVJ Hot Rod exceeds recommended tube manufacturer specifications.

On a JJ EL84 spec sheet the specified maximum plate voltage is 300 volts and the maximum plate dissipation is 12 watts.  Manufacturer specifications may vary for different tubes but most list these values.

Biasing the EVJ Hot Rod YouTube Video

Through the cathode flows the plate current and the Screen current (pin9), measure the current that flows through the cathode then subtract the Screen current.
10.5 volts is measured across the cathode resistor with a value of 225 ohms,  Cathode current  = 10.5 volts / 225 ohms = 46.6mA

To measure the Screen current a resistor will need to be added between the screen (pin9) and the B+.
To insert the resistor between the screen and the B+, the trace could be cut on the circuit board and a 1k resistor added.

Instead a pin is cut on a socket saver and the 1k resistor is connected between the pin on the socket saver and the pin on the tube.
Current = Voltage / Resistance    Ohms Law Calculator
4.4 volts is dropped across this 1k resistor, 4.4 volts / 1000 ohms = Screen current (.0044 Amps or 4.4mA). 

Cathode current  (46.6mA) - Screen current  (4.4mA) =  Plate current (42.2mA)

To find the plate dissipation of the power tube, multiply the voltage drop between the Plate (pin7) and the Cathode (pin3), by the current that flows between the plate and cathode. Plate Voltage 312 volts x Plate current .042Amps = Plate Dissipation (13.1Watts).

Most EL84/ 6BQ5 tube Manufactures Recommend a Maximum Design centre value (should not be exceeded under normal operating conditions)
300 Plate Volts and 12 Watts Plate dissipation, while allowing for 10% +/- for variation in supply voltage.

Some of the Capacitors and resistors have been replaced to improve the tone and bass response and increase the harmonics.

To Lower the plate voltage the stock 220 ohm dropper resistor (1) is replaced with a 680 ohm resistor.
To Lower the plate current the stock 220 ohm cathode resistor (2) is replaced with a 270r ohm resistor.

With these two resistors swapped - Cathode current = 10.6 volts / 270 ohms = 39.2mA, - Screen current 3.6mA = Plate current (35.6mA).
Plate Voltage (298 volts) x Plate current (.036Amps) = Plate dissipation (10.7 Watts).

No two power tubes are exactly the same. Some may move more current than others. Whenever you change the tubes you should check the bias.