Well... two steps back... but the path forward is now better.
I wanted to be sure the power amp wasn't playing a part in what seemed to be tone and dynamics deficiencies of the "ODS" circuit, so I bypassed it and played around again with just the power section to be sure, and there are indeed problems with the behavior of the power amp, all through the range of volumes. In order to adhere to the generally accepted good practice of keeping my calculated plate load line crossing the 0V grid contour line above the "knee" of its curve (see plot in Feb 18 post), I had to use larger resistors than one usually sees on the tube screens. Apparently, this produces "... a significant degree of signal level dependent negative feedback across those massive screen grids...", and the poor sounding result is no surprise to amp forum member Pdf64 quoted here. I changed a few resistors around to mimic some standard power amp designs and that really improved its performance in all ways. But I haven't yet gotten an answer regarding the problems those changes have now created in my load line plot! ("Sure, it may work fine in practice, but it will never work in theory!")
Other folks chimed in with comments related to my schematic: the V3 shared cathode causing problems with the stereo field, the placement and values of the R/L level pots, and one crucial rule of thumb from a builder that sorts out an issue that has been vexing me since the start: I have been making an assumption that for a clean sounding amplification stage - including in the power section - the design goal is to limit the max incoming signal to remain at or below the clipping point of the tube. It's logical, and might be true for a hi-fi music amp, but that's not how it works with guitar amps. This builder says he usually aims for allowing a max signal to the grid of 1.5x the tubes theoretical clipping point. So, if the power tube's bias point is -20v, organize your signal driver to deliver a maximum peak signal of about 30v (coincidentally, about 20v rms) to the power tube's grid. He says this gets you just into a distorted sound at max levels, but clean for most of the input range. This makes me feel a bit more relaxed about exceeding calculated grid limits, and gives me a new reference point for prototyping and modeling.
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