More capacitance isn't a problem there, put in whatever fits! The ripple current on each 100uF/450V cap is tiny, like 4mA p-p.
If you have the C4S upgrade, the 22uF/450V cap will be nearly inaudible. I have also run the C4S board right off the 100uF/450V cap and removed the 22uF/450V cap and 8.2K resistor on each channel. This would give you more space for the replacement of the 100uF/450V cap (like a 470uF/450V for each channel).