There you go then, your amp is biased by monitoring cathode current. You actually measure current, by taking the read across a resistor between the catode and ground. This resistor 'translates' your mV reading so to speak, it's a terrific and easy way to monitor bias in theory.
Unfortunately, as PBG said, many things can go wrong when you monitor current constantly - amp manufacturers stick too many components between cathode and ground, somewhere on the PCB, and they increase the number of things that can go wrong.
Of course, all sorts of thing can go wrong in an amplifier, but any design feature that can lead to your amp losing bias should be avoided. I've had quite a few amps on the bench the last years with these kind of issues.