Normally I would recommend 5.6uF to 27uF as the normal range for experimentation. But MonAmour uses a very high plate choke inductance, which can allow a smaller capacitance to be used without losing all the bass. It won't hurt the amp to go lower in capacitance. Roughly speaking:

1uF is 3dB down at 50Hz,

2uF at 25Hz,

3uF at 16.7Hz,

4uF at 12.5Hz.

This is the simple RC formula f = 1/(2*pi*R*C) where R is the transformer primary, 3000 ohms. For these small caps, the plate choke impedance is high enough that it can be dropped from the analysis, leaving only the capacitor and the load.

Of course, there will be interactions with the speaker impedance variations. By using a smaller capacitor, those interactions move up along with the cutoff. Just be prepared for some odd bass changes. Precise predictions are not possible - experimentation and listening are the tools of choice.