Most traditional carpet underlay and the new type of PU underlays are not suitable for use with this type of heating system. Duralay make a carpet underlay called Heatflow and this underlay has a low tog rating.
As a general rule the combined tog rating of carpet and underlay should be less than 2.5 tog. A high quality 50oz wool twist carpet will usually have a tog rating of approximately 1.5 so the underlay used needs to be less than 1 tog.
It is difficult to say you should go for the lowest tog rated underlay as comfort (softness) generally goes down with the tog rating.