Good news! 
We found out the problem with the latency, which was the Mainstage output. By reducing the sample rate to 32, we have ‘zero’ latency.
I have not done a full gig with the new set-up yet (that will be tomorrow and Friday), but I am extremely pleased with the way the MADI runs. The response / feeling from the keyboard samples running from the Mac is faultless: if anything, it will take a little getting used to, because the latency is so low. When we were running the MOTU896, the response was very good, but there is a very noticeable improvement. On top of that, being able to separate the different layers as tracks in the iDR32 means that the FoH engineer has proper control over the mix, without having to affect the in-ear that the player is mixing himself.
I will also try simultaneous MADI in and out, sending samples and recording the entire show in the next few days, and hope it doesn’t crash Logic / Mainstage! Perhaps Reaper would be a better bet for getting the WAV data in …
