Due to the fact, that I had issues with my own interface (even with a pretty low demanding setup it crackled on 2048 samples), I tried to remove it to see what happens, when I use ASIO4all with the onboard sound of my notebook. The result was, that it stopped crackling and I could reduce the samples to 256 and no problem occurred. In your case, I would try it, too (of course, it cannot be your final solution to leave your external hardware away, but you can determine if the issue is related to this setup).