Thanks, greenfly - good to see you had success for your setup, and thanks for your insights.
Are you using yours as a second interface with your UAD? I'm just going ADAT into the MOTU 624 and I honestly don't remember setting ES8 clock to internal under those conditions, but just establishing MOTU as ADAT master, and assuming ES8 hears and obeys.
I have the 8A on order as I need the additional inputs straight out- but haven't yet committed to ES6 until I work with connecting the two interfaces together and getting that stable (it's never as seamless as they make it sound, of course). I might even see how much I can use 8A's outputs to communicate with modular and VCV, etc.. since like your UAD, the I/O in the MOTUs is DC coupled.
Sometimes you never know how things will work until you start connecting cable - everything is just theory until then!