On the flip side (no pun intended) I've been riding road bikes for the past few months and for the first time since around September, I got back on my MTB/hybrid and I just feel like I'm sitting bolt upright, even though when I see my reflection in a building window, I can see that my posture is more like 70* from horizontal. The idea here is that once you've been riding your cross bike for a while, you'll get so used to being stretched out that your old MTB will feel like you're sitting with your torso perpendicular to the ground. Give it time and if you don't adapt, get a shorter stem so you're less stretched out. If the handlebars still feel too low, get a stem that raises them a little; a 15deg stem should do that.