I’ve got and Edirol FA101 and Motu 828 mk1 aggregate device, and occasionally the sound disappears when I add an instrument plugin (Uhe Diva) or similiar. It’s hard to figure out what does it, but it might be a buffer overrun or clock sync issue?
Happens intermittently, and randomly. Usually I fix it by opening prefs and selecting the FA101 by itself and then reselecting the aggregate device again.
I think this is a osx or custom driver problem. Try to change the clock master device in the audio/midi setup of osx. Are both devices USB devices? Then try to plug it either on the same hub or different ones. Renoise can nothing do about it.
Does it happen if you use the aggregate device with another daw?
I also experienced with firewire that a shorter cable usually works more stable, also the order of the chained devices seems to change something, too. But I also plugged a PCIe 1x firewire card into the computer with a texas instruments chipset (10 euro). So I don’t have any problems with aggregating devices anymore.