A3801 value?

[martin manning]

...the additional caps give the amp considerable more bottom end. JM said that it actually prevents upper frequencies from passing, but you can truly hear a bottom boost when you engage the switch.

You're hearing it right- like virtually all coupling caps it and the grid leak resistor of the following stage make a high-pass filter. Every time you double the size if the cap you lower the -3dB corner point one octave, allowing more bass frequencies to come through.