Cathode biasing is usually automatic, you loose a little power with this, that's why many use fixed biasing which can be adjusted.
George's comment is correct. If the amp were cathode bias, no adjustment would be needed.
Are the tubes stressed by being off the bias target setting? What would be good plus-or-minus numbers be for each? Why did one designer choose manually setting bias voltage and why the other chose manually setting bias current?
If the idle current is higher it will stress the tubes.
You are better off with too little than too much!
Adjusting the bias current and the voltage are really the same thing- the voltage sets the current through the tube(s). What you are measuring is either the current itself or the voltage that it induces across a resistor, but either way you are measuring the same thing- the correct current through the tube.
Thanks George, went to the site and thanks atmasphere. Perhaps a embarrassing question: do I assume that the absolute value of idle current drawn across a resistance would be polarity-neutral, (whether or not there is a minus sign in front of it), making -.64 volts a "higher" value and drawing more current than -.55 volts?