[Ron Ramirez]

Perhaps the less than optimal audio quality is due to the fact that the C8-20 has a single ended 6F6 audio output and the 37-640 has a push-pull output?

But that isn't really correct either, since Philco resorted to "cheating" to get a push-pull output without the use of an audio interstage transformer or a phase inverter tube - they tapped an out of phase audio signal from one of the 6F6G output tubes and applied it to the control grid of the other 6F6G tube.

I would still suspect something not being quite right in the C8-20. As I mentioned, my similar T7-5 plays well and the sound quality is very good.

(I meant to type T7-5 in my previous post; I have now corrected it. Although I do also own a T8-14.)