Another aspect I'm thinking about is the method of coupling from the first AF amp to the push-pull 1AG4s. The primary impedance of the interstage transformer assumes that it's driven from a "10 k" plate, meaning a triode. Screen-grid amplifiers have a much higher output Z, and so in coupling your screen-grid first-AF stage to the push-pull output stage via that transformer, you're beginning with mismatch loss to the transformer from the 1AD4 screen-grid-AF plate. (In more-usual circuits, a triode driver would be used; or RC coupling would be used from the pentode-AF-amp plate to a single-ended AF power tube. In this case, though, rewiring your first-AF 1AD4 as a triode to better drive the transformer may give more gain, the same gain, or even less gain, "depending." And of course going to a single-ended output stage would halve the possible available output from that stage.) That said, the 1AG4, with its specified output of only a few tens of mW, was likely intended only to drive headphones. There are subminiature tube capable of hundreds of mW output, but headphones output is better for two-way work.