Mbox Distortion

[AMcG]

Hi,
When using my Mbox mini I am hearing clipping/distortion even when working at moderate levels. It does go away if I turn the output down past 12 O'clock. If I use the built in output, then the sound is fine. So it is possible to mix by bypassing the Mbox all together, but this isn't a practical long term solution. When the problem first occurred, I could make it go away by changing the "Playback Engine" settings a couple of times. Nothing specific, but changing a few settings and relaunching Pro Tools sometimes worked, but even that doesn't do it any more. Could this just be a problem with the Mbox? (It's an mbox2, probably about 6 years old). Would it be worth trying to borrow another Mbox to test with my system?

Cheers.