Marshall Vintage / Modern 2266 50 Watt High Gain Option

These are all adjustable bias amps and all you need to check and adjust the bias is a Multimeter and you can find all these under the Related Products Tab. 

These amps have internal test points. They look like three pins, the middle pin is ground, the right pin is for the right tube and left pin is for the left tube. Right next to the test points is the trim pot, it controls the bias for both tubes. Set it so that each side reads around 48mV. We usually like to see each side within 2mV but it appears that there is some error between the two test points so as long as they are within 5mV you will be fine.