If you buy an expression pedal you're pretty much limited to just a wah... I would suggest putting your rp100 on Ebay and shelling out for a GNX3 or GNX4.... I have a GNX2 (same as 3 minus the recorder) and it gets the job done. You can control a bunch of different parameters with the expression pedal not just the wah.... you can control volume, gain, wah, effects level(phaser, flanger, etc..), delay timing, reverb, and a bunch of other stuff. That's my suggestion