I recently had a very similar experience. My idea was to use IEMs with ambient ports for practice, with the dual benefit of hearing my keyboards well and reducing the high volume of the band entering my ears. (It's gonna be great, I tell you!)
I bought the less expensive Westone AM10s to try it out. I don't play much low register, given the band setting, so thought the single driver might be suitable. And at home it did sound reasonably good.
But at practice, it was a disaster. Funny thing, though, I'm not sure how to describe why it was so bad. Somehow both the keyboard through the IEM speakers and the band through the ports sounded heavily distorted. I don't quite understand why. Wish it would work though.
I'm hearing you. On the distortion you mentioned, am getting the same thing plus some sounds just don't sound like they should. Example: patch is a piano sound - out of FOH it sounds ok but to me through IEMs it sounds like an out of tune honky tonk piano. I'm assuming all the other noise is somehow changing my perception of the sound.