Did you ever solve this?
I just got my Pod Express and I'm having the same issue, I can't get a proper clean sound. The best I can get is to lower the gain as much as possible and turn down the volume knob on my guitar. To get enough volume, I max the channel volume, master volume, and cab volume. This way it's decently clean, but still distorts sometimes if I play too aggressively.
I thought my unit was broken at first, but looking up Fender Princeton on Youtube, it seems like it does indeed always have some distortion.
Firmware is up-to-date, and I've tried this on a variety of setups, with active and passive pickups, monitoring with different headsets, and through line-out and USB. They all have the same issue.
Very strange indeed by Line6 to not include ANY pure clean sound in this.