I used what I'd call a sub-compact double when I rode the ST. It was actually a triple with the big ring removed. Mine was 39/26 and it worked out very well.

BTW, I set up the chain line for good alignment with the big ring not worrying that the little ring was off a bit since I only use the little ring with a couple of the largest cogs.

With the current crop of super wide range mtb clusters... If setting up a new bike, I'd consider a 1X10 or 1X11 and get rid of the front derailleur altogether.