Well, Mackie it'll be. 
 
The USB option was only a concern because I was looking for the simplest form of 2-way connectivity. With that out of the way, having taken the time to mentally reconfigure my set up and its cabling, and think of every possible scenario I could imagine, the Mackie emerged as the best option for me.
 
The Allen & Heat was very tempting, but a few things made the difference - Mackie has faders and a 18db low cut at 100Hz. The ZED has a gentler 6db one, and I prefer something steeper.