I am going to respectfully disagree with the above comment that disparages upgrading your amplifier’s coupling capacitors. Tube amplifiers in general and particularly SET amplifiers have very straightforward/simple circuit layouts. With regard to the SET amplifier there is a lower part count.
Essentially one could argue that because of this, each part individually takes on more importance particularly in the audio signal path. I believe that changing to a higher quality coupling capacitor is a very good idea and money well spent.
if you take the time to research and do further reading/investigation you will find a better capacitor that will contribute to improved sound quality. I’ve had splendid success with the Duelund CAST copper foils. Considerations are available space for a given capacitor and the cost. No doubt there are other excellent choices available.
I suggest that you reach out to @grannyring via site email. He is most definitely a knowledgeable and helpful resource.