And just to clarify, you can use the older 48k boxes along with the newer 96k boxes, but each style of box will require it’s own dedicated connection to the console (ie you cannot connect/daisy chain the two types together). Since the Avantis only has one connector built in, this would require the use of an appropriate I/O card.
In your example, you could plug the GX4816 into the built in GigaAce port and then plug the AR84 into a GigaAce I/O card installed in the system. Obviously it’s probably not worth doing that as purchasing the I/O card is only slightly cheaper than purchasing a DX168 box that could be plugged directly into the GX4816 (thereby saving the I/O slot for another use), but it is a solution if you really need to use both 96k and 48k boxes at the same time.