That once again backs up the statement: In well designed amps, the similarities between tube and solid state or class A, AB, and D are far greater than the differences.
I want someone to build a class D, tube amp just for fun. You'd just need a high frequency triangle wave generator, a comparator, power valves which could handle the switching frequency, and then some capacitors to act as rectification filters to remove the switching component of the signal. Although, I fear the frequencies required are beyond what tubes can handle.