There tends to be too much concern about the power side. Of course it is not the best idea to pair the Explorer with a 600 Ohm can, but that is really an extreme example.
Frequency response behavior is of much more importance. On goldenears you get technical details about impedance over the frequency spectrum. Remember that the Explorer2 is an almost perfect source with almost zero source impedance. A lt of Sennheisers have an impedance peak at about 100 hz.
I recommend an impedance, which does not vary remarkably with frequency.

Hope that helps