• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Independent Bias Adjust for Each Tube?

That's about it: balance the currents on both sides of the center tap, minimize residual DC core magnetization. It's especially important when using toroidals, since these have no gap whatsoever and are highly prone to core saturation.

Without that DC mag getting in the way, the bass performance of the OPT improves since the core isn't so easily saturated.
With separate bias controls you don't need to have the output tubes perfectly matched. Most guitar amps use cathode bias so matched valves are mandatory. I was told by a veteran tube man, now passed over, that the trick was to set volume to minimum then adjust each bias control for the required tube current it controlled. After this, tweak one of them very slightly for minimum 120Hz (100 Hz in UK) output on the speaker terminals to achieve current balance. That's the core saturation bit as described in the previous post ;)