Rodman, negative feedback (mandatory in a push-pull pentode amp) will offset 90 to 95% of the transcoductance delta.
You are 100% correct regarding 12a_7 tubes in zero-feedback SET amps. These must be matched if you want a central stereo image.
You are 100% correct regarding 12a_7 tubes in zero-feedback SET amps. These must be matched if you want a central stereo image.