The Central has only line-level and digital outputs, not speaker outputs. That means it was meant to be plugged into a stereo system that's already got an amplifier with its own volume control. In fact, I'm curious as to how you're even amplifying it in such a way that you don't already have another volume control for it.

To answer your question directly:

- You could write your own Linux/ARM player software from scratch which includes a volume control.

- You could figure out how to hack the kernel to include some kind of rudimentary volume control, although integrating it into the main player UI would probably prove to be quite tricky.

- You could purchase and wire up a stereo potentiometer on the RCA output cables.

- You could hack the stereo potentiometer into the casing of the unit.