I’ve been emailing a bit with MOTU tech support. I am now able to get WaveLab to playback with my MOTU 8D without needing to make an aggregate device.
I think the problem was that when you install the MOTU Discovery apps and all their fun stuff, the Mac sees the device as both a basic USB class compliant device, and also as another device via the MOTU apps. I kept seeing two different 8D devices listed in Audio/MIDI so I went any manually uninstalled all the MOTU apps and now only one 8D is listed in my devices.
Here is a PDF with more info:
http://techlink-attachments.motu.com/tickets/117361/MOTU%20Pro%20Audio%20troubleshooting%20macOS.pdf
You can still access the mixer settings on the web by entering the IP address of the interface if it’s connected to the computer via Ethernet.