Difference in Demands on Amps

All things being equal, will a Magnepan 20.1 or 20.7 present a more challenging 'load' to your mono-block amps? How does one really quantify that difference if any?
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My question didn't come out quite right; what I meant to ask, is there a difference between the load that a 20.1 or 20.7 would put on a set of amps vs. a pair of Magnepan 3.6's? Are the 20's a significantly more demanding load than 3.6's?
I think there was a thread about a year ago from someone who was having trouble driving the 20.1's with the Cary 500's. If I recall correctly, it was pointed out that the 20.1's had a pretty significant impedance dip to about 2 ohms that you don't have with the 3.6's.
I'd not want to speculate on the 20.7's. The crossovers are different. I'd want to see the impedance curve.