Veils functions similar to the Quadratt EXCEPT you have CV control of each output volume. I wouldn't define either of these as a proper mixer, though you can use them as basic mixers or submixers. I would think about all of your outputs (drum modules, FX units, etc.) and figure out how many audio signals you might want to mix together at once. With eight "mixer" inputs (seven if you're cascading one unit into another), you're going to probably be short when it comes to a complicated patch.
Look into the Roland 531, Happy Nerding PanMix, Pittsburgh Modular Lifeforms System Interface, and Blue Lantern Stereo Sir Mix A Lot. All of them are 6 input mixers with pan capabilities on all/most channels depending on the unit. With exception to the Blue Lantern, they all have headphone outputs as well.
You probably won't need a decent mixer to begin with. The Quadratt and Veils will be enough to get you going. But as you start to flush out your case, you'll probably want one.