Replacing elements in a tube amplifier
C9: think of it as a low-pass filter: the higher the frequency of a signal, the more it will be "shorted" through C9, and the less it will be coupled through Tp2.

If you increase C9's capacity, you'll cut off at a lower frequency.

C8: AC-couple high-frequency components to ground – its function isn't 100% clear to me from the schematic. I wouldn't change it – that's there's a potentiometer right next to it.