1.25" to 1.5" is the proper height to make sure you do not have an issue with cabinet/vent loading. This is the required amount of space to allow the driver to displace enough air that it can properly go through compression and rarefaction. If using the Outlaw feet, you are fine. The Ultra is a very tight sub, as the internal bracing eliminates most cabinet resonances and the Class LH amp tracks the audio signal - if you are experiencing 'muddy' sound, I would also recommend an isolation pad to decouple the sub from the floor.