Not sure why, with you're completely digital system the need to mix down through an analog console?? You could still mix down using just the RME interfaces 8 line outputs by creating multitrack stems in the box that would then be combined and fed out from 1 to 2 outputs per stem. So you would only be mixing down 8 analog tracks through your console. Nothing wrong with that. You are already using a hybrid system and so you shall remain hybrid.

Conversely, if you like to mix down through analog consoles, why not sell your 2 different digital audio devices and purchase yourself an Alesis HD24 for under $1500 US?? Then you will have a digital recorder with 24 analog tracks in and 24 analog tracks out, all with decent converters, with the ability to remove the hard disk drive drives from the recorder, connect it to the FireWire interface, dump into your computer, manipulate the tracks in software while adding equalization, effects, compression and other DSP then, dump back to FST hard drive, put it in the HD24 and mix down in analog after having done all of your DSP processing in the box first?

You have started out kind of backwards, thank goodness there is eBay!
David Ann Remy Ms.

Since the Digimax is a preamp ADC, and not really a DAC interface, yes, you will need something else to run more channels to your console.

I would just add a D/A converter - RME ADI8 Pro or DS would work well here. There are other options of course (any ADAT/lightpipe input DA would work). You could run 16 additional channels of DA from the Fireface along with its' own 8 for a total of 24.

Perhaps you might want to purchase a damaged ADAT machine to use just for an eight Channel, light pipe based analog-to-digital and Digital to analog converter? You can probably find a broken one for $75 that won't pull tape? Perfect cheap stuff to get you started.

Cheap broad
Ms. Remy Ann David