Yeah, part of why I haven't tested one. I don't personally have something that would be a good match with it and I don't have access to one that would pair well in order to see the difference. This is one where I'm just gonna have to wait til I can try one out at a meet one of these days.
Luckily, with the HD600, it either sounds fantastic with the right setup or slightly less fantastic with any other setup. I haven't heard a setup yet where the HD600 sounds bad; it can only sound less good compared to ideal setups. Which is still quite good