Matt would you please tell me about the grid stopper (10K) and what it does and how it works and what effect on sound and amp?
The short answer is that the grid stopper limits low level bias excursions in the coupling capacitor.
Without a grid stopper even very short duration bias excursions can cause clipping and give rise to odd order harmonics in the output. This problem is then exacerbated when the grid leak resistor is large because this lengthens the recovery time making the odd order action more apparent. With the sizable grid stopper in place, bias excursion take place much more slowly. This converts the clipping action to more of a compressive action for small excursions therefore replacing the discordant odd order action with a more pleasing even order action. This means that with real music (with typical instantaneous dynamic range), the amp can be run closer to max without discordant effects. This is one of the reasons that when listening to real music, a well designed tube amp sounds louder than a transistor amp at the onset of distortion. The perceptible distortion onset takes place only well after small bias excursions are already taking place.
This topic is really worthy of a much longer discussion. Perhaps when I'm done working on my current design article for the web page, I'll do a short article on grid stoppers, bias excursions, and the effects on amplifier sound.