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The 6BM8 will also work as the input tube, as its pentode will serve as the voltage amplifier and the triode will work well in the Split Load phase splitter. (I want to build this version sometime soon, as a 2x3 array of these tubes on a printed circuit board would certainly be clean looking. Additionally, using all the same tube has the advantage that they can all be tested for noise and the winners can be placed in the input tube position.)
If a discrete pentode, such as the EL86, is used for the output stage, then a 6DJ8 or 12AU7 type tube can be used in the Cathode Follower position. The 6DJ8 or a 6AQ8 will also work perfectly as an input tube and phase splitter.
If you wish to keep the amplifier looking more like a Futterman, then an EF86 or 5879 would be a solid choice for the input tube. One trick would be to feed the output directly into grid 2 of the input tube. With the output stage configured as a unity gain, non-phase inverting buffer, the output signal should equal that on the plate of the input tube. In other words, the output signal will force the input pentode to function in triode mode, a mode that intrinsically entails a form of feedback. This is a marvelously short and direct feedback loop.
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