Hi Dave

Yes the Sound Devices mixers send out a line level signal, so the EX1 inputs should be set to line-in. The EX1 audio is so good you almost don't need a mixer, but a mixer does give you total control and awesome limiters in both the input and output stages.

I get excellent audio when doing this:
 

I have ex1 and sound devices 442.
1. Send test signal from 442 (normal settings-have not change loudness of the signal)
2. Put one channel to auto and other channel to manual in ex1
3. Change manual channel to same as auto.
4. Change auto channel to manual and match its volume. (for me it is little over five in ex1)
5. Change test signal off
6. Drive Audio to mixer so that it is quite near where green changes to yellow area in the mixer...maybe little in yellow.

This way signal should be very good and mixer uses its compressor/limiter if driven too hard.