The issue with putting ample substrate over the uth is two fold.
1. The extra substrate makes it impossible for the heat to penetrate it and the snake does not receive the heat it is sending.
2. The extra substrate insulates the uth and traps a lot more heat. This is an issue if your snake decided to burrow down to the heat source it will be in a zone that is way to hot and can cause all sorts of problems. Don't think that just because you have not seen it burrow that it wont. Mine ball will root up the entire tank from time to time for what seems like no reason.
A good tstat is cheaper than a vet visit. I would unplug the uth as it is not doing anything for you right now.
By ample, I don't mean a mountain of it. By that i mean that i can stick maybe 1/4 or less of my finger into the substrate then hit glass. Maybe i misused the world "ample"