Dont really know. I have read that an attenuator can be straining enough even when its matched properly. With a load mismatch it sounds like it would be dangerous. Bob
It should be fine. Lets say you are attenuated about half way to 8db or so. 50 % of your amps "sees" the 16 ohm load of the hotplate the other 50 % "sees" the correct 8 ohm load. Its never going to "see" more than 16 ohms and should be safe given that 16 ohms is safe to begin with.