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In other words, because the triode's cathode finds a low-impedance path to ground through the bypass capacitor, the triode doesn't see its cathode resistor's value in its AC operation; in its DC operation, however, the grid's DC voltage and the cathode resistor's value set the idle current through the triode.
The feedback loop encompasses the output coupling capacitor and purposely allows the DC voltage from the first triode's cathode resistor to be relayed to the other side of this capacitor. This small positive DC bias allows the use of a higher valued cathode resistor for the following gain stage's input tube, which helps us realize a lower gain from the second gain stage without having low-valued feedback resistor drag down the output too severely.
The last coupling capacitor is needed to shield the volume control and the line stage input from the DC voltage present on the feedback loop (potentiometers should never see a constant DC current flow through their scraper).
In the circuit above, we see that adding a cathode follower eliminates the need for the last coupling capacitor. The 20k feedback resistor is now is no longer in parallel with the plate resistor, which will result in higher gain from the that triode because its plate is sheltered by the cathode follower. Now the total coupling capacitor count has fallen to two and we have retained the feedback loops, while improving the stability of the amplifier. Not bad.
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