Usually, when you're going from "consumer" gear to "pro" gear, it's not just a cable, you will want to make sure the levels are matched, and that you are wiring the correct pins (sometimes you want to wire hot-to-hot and ground-to-ground, but sometimes you want you to wire it ground-to-negative...I think...don't quote me on that). My Mackie manuals typically have a wiring diagram under a section they call "connecting from an unbalanced source". You may want to look for a similar section in the manual for your headphone amp (what kind is it, anyway?).
I forget what the conversion is. Someone here should know.
OTOH, if it does end up just being a cable, Markertek usually has freaggin' everything.