Ever try just not heating the sauce? The jar stuff is already cooked to death. Warming it up again kills any flavor left. I stir in fresh herbs right at the end, off the heat. Basil or parsley. It keeps them bright and fixes that flat taste. Makes a huge difference.
Understand completely, that flat taste just kills it for me too. I keep a tube of garlic paste in the fridge for emergencies. Squeeze some into the sauce while it heats up, maybe a pinch of red pepper. It's not perfect but it gives that jar a much needed kick. Makes it feel a little less like you just gave up on dinner.