There is a whole lot more to it than fixed or automatic bias. For a start, with AB:
Is it a common cathode resistor?
If so does it have a decoupling capacitor?
If not, do the individual valves have decoupling capacitors?
Is the bias regulated hard?
Is there some degree of compensation for HT variations?
Is it just a cheap resistor network? Yes I have seen that!
Is it low Z?
Is there grid current?
Are the drivers capacitively coupled ? (can temporarily bugger up bias if grid current is allowed)
Are there separate phase splitter and low Z drivers?
When looking at valve bias you absolutely must include the drive conditions. There is no point in having a precision setting regime if at the slightest hint of overload, rectification by grid current backs off the bias by 10V until the coupling capacitors and bleed resistors balance the charge.
Mind you, we are talking guitarists here, so maybe that's a form of distortion and 'shunting' effect they want
It wasn't me!
(Well, actually, it probably was)