I'm going to be running this project at 18volts via the Roadrage, but the only cap I have rated above 18 volts is a 220uf. I'm pretty sure this is an OK substitution for the 100uf (I think its called a DC filter cap) but if anyone could confirm I would be much obliged.
ya, it has no effect on the signal path, you can make it as large as you want (although I won't recommend going smaller)
it's purpose is to filter out as much ripple from the power supply as possible.
Radical, thanks misha!