Hi-
I have a BX1860, which is similar to your BX2660. I don’t think you’ll have to take the loader off, but you will need to remove the brush guard. You won’t need to remove the upper bonnet, but you will need to use the plastic strap/holder that holds upper bonnet half-way closed. If the upper bonnet is all the way open, you won’t get the lower bonnet past the headlights.
You’ll then want to angle the open end of the lower bonnet slightly down and around the upper bonnet and slide it down and around the engine. It’s not what I’d call easy , but it’s also not hard. I sometimes wonder if Kubota went out of their way on the 60 and 70 series BXs to make this particular aspect a pain.
Hope that helps.