That DBX 386 has spdif digital out…you could connect it directly to your MR for an extra 2 channels.
There is no decent cheap option to convert line signals to ADAT that I know of…you might instead consider something like the Behringer ada8200 which will give you another 8 mic pres to ADAT and then if you want to try standalone preamps later you can just plug them into the mic inputs.
Note that the preamps in the 8200 are supposed to be very useable.