A.M.
We also have a stacking setup, and I have a baby with very sensitive skin. I find that All Free & Clear is reasonably priced and it says HE on the bottle. I've been very happy with it. Tide's dye-free perfume-free version (not sure what it's called, maybe Tide Free?) also makes an HE version.
You should know, though, that you can use any type of detergent. Just make sure that if you bought one that doesn't say HE on the front, USE LESS OF IT. When I buy a non-HE detergent like Seventh Generation I use almost half of the recommended amount. It works out just fine.
I also set the washing machine to always perform an extra rinse cycle so that all of the detergent comes out of the clothes before they get dried. If your baby has sensitive skin, you should do this too. Not very water-wise but better for baby's skin.