I've had one KT88 fail in the past 40 years and it took out the surface mount autobias board.
I'm probably not the typical tube owner as I have an hour meter and test and reseat tubes every six months. [about 500 hours]
I must stress that there is more to keeping tube gear going that just bias. A failing tube can either pass too much or too little current. When bias is checked, you should note the existing value, the datetime and the adjustment in a log.
If one tube is going in a different direction than its compadres, it's probably time to take action.
As far as those that claim bias is the only thing that matters, BUNK! You might get away with it in a single ended amp, but push-pull requires equal current change for equal grid change. If otherwise, the waveform is distorted.
And for all you rollers, you can change the sound by accenting or negating mismatch in the driver circuitry with mismatch in the power tubes. SO MARK YOUR TUBES SO YOU CAN PUT THEM BACK IN THE SAME PLACE if you really like the sound!!!
If you have multiple power tube pairs per channel and if you can test your tubes for Gm, you can balance the pairs to improve distortion. AND the match info written on the box ain't necessarily the same as it will perform at different voltages and currents. AND almost certainly not after a thousand hours.
