Head and Rack support 2 channels in plus 2 channels out, while Stage support 4 channels in plus 4 channels out.
I suggest to read the few pages related to OS 9.0 and USB audio in the Addendum 9.0.
The Main Manuals in English, Japanese and German are also at 9.0 state.
I wish, wish, wish(!) that they'd upgrade the hardware on the toaster to incorporate cool stuff like this. It'd be up to the discretion of existing users to buy a newer unit if they want these features...which I don't think is too much of an ask.
Likewise, it'd be a template change and a change of component to include a sturdier network connection like the one on the remote. And WIFI. I really don't see why they won't make these subtle changes to the hardware to make them staples for years to come, it's not as if these small things can't just sneak under the radar like the clock rate change did a couple of years back, for newer units only. I think I heard that the Stage boots up faster, too! Real shame.
Yes, I could buy a Stage, but I dislike the form factor for studio use (my primary use).