It sounds like back heat to me.

If you are using it as back heat you are going to have a lot of heat loss-there's way more air exchange with back heat than with belly heat. Can you turn your heat cable up anymore than that?

Also, the holes in tubs aren't for heat, they are for humidity. The more holes you put in the tubs the less heat you are going to hold in the tub, as well as less humidity. Holes are generally, if not always, put in the front of the tubs.