Hello Owen:
If you are happy with these interfaces, the easiest solution is just to swap them with working units from the 2nd hand market. Here is great deal on two:
https://tinyurl.com/yckj3yfe
However if you want to upgrade, as most modern audio interfaces are USB/2, 3, or C then I would recommend the MOTU 24Ao which actually gives you 24 channels of outputs. Using the provided AVB network connection, you can add 2 additional 24Ao units for the max limit of 72 channels. If I remember your setup correctly, you have a large multichannel setup correct?
Regards,
Danny B.