I actually have the same issue. I was wanting to test the Helix software running from my Macbook pro using my HD500x as an interface for both input and output. However, I'm not confident I can do this because I run into this issue using plugins in Garageband. Currently, I'm using a "dry" patch to feed into my macbook to be processed by plugins I run in GarageBand. However when I do this and attempt to feed the 1/4 output of my HD500x to a monitor, I get both an affected signal and a dry signal. Same thing happens when I use the headphone output on the HD500x. However, when I use the headphone output of my Mac, I get only the affected signal, which is what I want. I don't know why I can't shut off that dry signal in my HD500x. I'm sure it's user error, but my googling hasn't found me an answer quite yet.
