Digitally Controlled Lowpass Filter

For a while now, I've been thinking it'd be fun to build a lowpass filter with resistance values controlled by a digital potentiometer chip. While googling about, I ran into a simple implementation of the idea on Maxim's site

The schematic called for one of Maxim's dual 10K digipot chips. I only had a dual 100K digipot (MCP42100) on hand, so went ahead and built a version with that, figuring I'd only use the last 10th of it's range - or augment the pot's value using an 11K resistor in paralell.