I assume that's for me -- not sure why it would be particularly unstable. It is just a direct coupled SE amp, which is easy enough to build and stabilize, driving a 300B. And, 6W6's are a dime a dozen, so who cares if one runs away ...
With two cascaded CCS loaded stages, gain is not only very high and "unswamped", but parasitics will be all over it like a dirty shirt. A snubber in each anode will likely be needed.
The snubber can be a 100-470R in parallel with a small inductor, or a few ferrite beads in the plate lead.
I played with very similar circuits (matter of fact, my headphone amp is a CCS loaded parafeed) and we had RF all over the place in every situation
Cheers!
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