heat pads - direct or indirect ?

Hi All,
 
When it comes to heat pads (or bottom heated propagators, etc), and using small medium such as Jiffy pellets, have you noticed better results when the heat is not directly applied ?
 
For example, in my bottom heated propagator, which is basically a heated bottom tray with clear plastic lid and aeration sliders, my Jiffy pellets produce results much more quickly when the pellets are actually raised off the bottom surface.
 
Maybe the pellets get a bit too hot when they are in direct contact with the base?
 
I also note it is easier to keep the general temperature up inside the propagator when the pellets are not in direct contact with the base, and this much makes sense I think because the pellets don't then act as a heat sink.
 
Anyone else notice better results with indirect heat ?
 
Regards,
 
Tim

I use a heat mat that does not have a temperature setting, so if it gets too hot I put a small towel over the mat to lower the heat.

Mine has no heat senser so I use a meat thermomator to make sure it stays the right temp. For me a direct is better. I use solo cups.