There are a lot more things that you could vary beside axial tilt. You could decrease the amount of solar energy reaching the top of the atmosphere either by increasing the distance to the sun or decreasing the temperature of the sun. That would lead to overall cooler temperatures. You could mess with the composition of the atmosphere, making it more or less transparent to radiation of different wavelengths. For example, invent some otherwise harmless compound that causes the atmosphere to reflect more solar radiation back into space. This would make the average temperature cooler, all else being equal. You could even vary the concentration of this compound latitudinally, so that the mid-latitudes actually absorb more energy at the surface than the tropics.