The tube input stage is the only difference as far as I know.
Schiit may have tweaked the parameters of the Vali's solid state output stage to better accommodate the tubed input stage. So the output stage on both amps may or may not be exactly the same.
But conceptually, the amp is a volume pot, input stage, output stage. The only difference being the input stage, and whatever adjustments to the output stage.
Maybe its just me, but I find it so interesting changing the input stage altered the sound as much as it did. While not a huge difference, I did find it easily noticeable.
I'll have to do a little more digging, as the tech side of things seems to be calling my name lately.