Yes you could use this to connect to many ada8000. The only thing is that you're going to be doing a lot of conversion there.

Firstly the sound from the sytnth (or whatever) is going to be converted from analogue to digital (which technically is two conversion - one to a digital signal and another to a an adat/optical signal) then the same conversion in reverse at the sound card, to the computer then back from the computer to the soundcard and converted in to an analogue signal for your speakers.

Having said all that, M-audio converters are generally very good, and by all reports the ADA8000 does a good job too.

Being a purist, I would suggest going for a Motu 24io, because if you think of the profire costing you £299 plus 4 x ada8000 at £189 per unit you're looking at £756.

For that money you could pick up a second hand Motu 24io (with a bit of hunting - there's one on ebay for £900), which will be sooooo much better than the M-audio/Behringer set up.