Always under your substrate, on top of the tub/tank.

You want the HOTTEST part of your whole enclosure to be no more than like 95.

Granted it's not going to be 95 on top of the substrate, but you animal can burrow down to the heat if he wishes.

Make it 95 ON TOP of the substrate, and the floor of your enclosure under the Aspen will easily be over 100 degrees. Which is just asking for a burn, and other problems that won't be any good at all for you.

HTH