Aiken sounds like a smart guy. I am not sure what to do. I am leary to set my amp to any other setting than matched impedances. I am leary to change anything that will take away the sound of the original marshall era design. I am guessing these amps really did eat tubes and do other bad things. Sounds like a bad design to me!
http://www.aikenamps.com/Biasing.html
In some cases, if the voltage is high enough, there is no bias setting that will result in safe operation without exceeding the maximum plate dissipation of the tube. For example, in a 100W EL34 amp with 480V on the plates and a 1.7K primay impedance, there is no bias setting that will keep the output tubes from redplating at some point in the power curve, even if you bias the amp at 0mA! If you double the primary impedance to 3.4K, however, the amplifier will operate fine at all power levels, and you can bias it all the way up to 52mA without ever exceeding the plate dissipation at any point in the operating curves. This may sound odd, because the bias point is right at 100%, but it works because of the very high plate impedance load, which never allows the tube dissipation to increase above the idle level. It also may not sound as good, because the screen voltage should ideally be decreased so the loadline intersects the "knee" of the curves if not, the nonlinearity increases drastically. The bottom line is that you have to take into account not only the plate voltage and plate current, but also the primary impedance to find out the safe bias area.
I don't doubt Aiken though but it is NOT normal for the tubes to red plate.