is it just a stereo pair in and out on the subzero?
No, it has 8 mono ins, make into 4 stereo if you like. Plus some 3.5mm jacks. Outputs is just one stereo out I think and headphones and class compliant usb. It’s a great little thing, I only paid about £60 for it 5 years ago. Compact too. It was a bit redundant when Digitakt got class compliant usb, but dig it out recently.
8 mono ins via usb??? (i’m only asking about the usb part…I can use a portable backpack mixer w/ class compliant usb)
No, 8 analogue ins (1/4” jacks) - the main stereo out (1 or 8 analogue in) goes to the usb class compliant port.
But what i’m asking is there not a stereo input on that USB (allowing direct output from the ipad like a sound card)
Nope, for that you’ll need a sound card, even if just a small device such as iRig 2. USB will do MIDI though, so need for a MIDI interface at least.