And so if I wanted to also use a digital drum, like say an Alesis Perc-Pad, as input 9 I could run that IN S/PDIF? Not at home now to check, but I assume the DAW would then recognize that via the USB connection as a 9th input? Can't imagine running USB out and S/PDIF out simultaneously, so I guess it would "have to" see this as a sep. 9th input run back out over the USB signal?
No man, I'm not sure you're getting it.
USB/Firewire/PCI/Thunderbolt is the way your audio interface talks to your computer.
It doesn't relate to analog audio or digital audio formats at all.
Your computer sees an audio interface (via usb) which has the following inputs ports available.
8 analog inputs (mic preamps - 8 physical connectors)
8 channels of digital input over adat (one physical connector)
2 channels of digital input over spdif (one physical connector)
The computer doesn't care what kind of format or connection you're using to get the audio to the interface.
The computer just sees input 1 or input 9, or whatever.
If that happens to be analog of spdif, it doesn't matter.
Now, what does matter is that adat and spdif, being digital audio formats, require the source device to have that format of output available.
Think of it like the audio is transferred via code, yeah?
To use an spdif input you have to have a device which sports an spdif output.
The reason I suggested a two channel interface as an example is because spdif carries two channels of digital audio.
Your computer will see it as input 9+10, or input 17+18. (not certain which)
The same applies to adat. If you bought an ada8000, you could plug 8 mics into it and run an adat cable from it to your interface.
Your recording software will now have access to 16 channels with mic preamps..See?
The built in preamps will be listed as 1-8 and the ada8000 ones will probably be listed as 9-16 or 11-18. (again, not certain which)
Even right now with nothing plugged in, your recording software should tell you you have 18 input paths.
Is this making sense at all?