the other reason for the MADI format difference is some manufacturers using the last '8 channels' for control or other data streams instead.
you want one of these:
http://www.rme-audio.de/en_products_adi_648.php
If will convert the ADAT to MADI and still duplicate the ADAT inputs to the ADAT outputs. So basically this will sit inline between the pre's and the stage rack and give you a MADI split out to our Mix Rack.