I just got the iostation 24c, using it with pro tools. I get digital distortion all the time. I may have found a way to get it to work, but it's a big hassle. I have to change the play back engine in pro tools to the mac speakers, then back to the iostation. Then it seems to work. I had been using the AR12C and never had this problem, but had to move it elsewhere to use.
I tried changing word clock settings, but the iostation doesn't seem to allow an external word clock. And in pro tools, I don't have the option under setup>hardware to change word clock while using the iostation.
I love presonus gear, but, So far, not super pleased with the iostation.