So I've just got a Quantum 2626, and I'm having an ADAT issue. I'm plugging a Focusrite OctoPre into the Quantum - sample rates match, cable is in the right ports, Quantum works fine on it's own.
In the past, when I've used ADAT, I've always synced the interface wordclock to the ADAT device - but if I do that with the Quantum, the light blinks red for a bit, goes back to solid blue like it's synced... and then nothing works. No input (the lights on the gain knobs don't even light up if I plug a mic in), no output. The OctoPre still shows input, but it never makes it to the Quantum. If I try and switch it back to 'Internal', it just flips back to 'ADAT' - I have to turn the interface off to get it back.
Weirdly however, if I leave the clock source on 'Internal', both devices seem to work fine - Quantum behaves as normal, and it gets input from the OctoPre. I'm suspicious of using it like that though, in case there's sync issues during recording.