So my sense is to consider the full impedance curve, not just a static number. Maybe that is where "all other matters" are not equal.
That is certainly part of it! There is a reason some speakers are considered 'difficult loads' and that is never a good thing as far as the amp is concerned.
The OP was about tube amps that can drive ML ESLs. The issue here is that ML has been really trying hard to make their speakers work better with transistors, which is not easy to do, because ESLs want constant power and transistors are constant voltage (see http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php.
I'm pretty sure I explained all this earlier- a lot earlier:
http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1394129996&openflup&11&4#11
But if you have a set of ZEROs, its pretty easy, then its just about whether you have enough power, not whether the amp is comfortable with the load. Seems to me 60 watts will make most MLs play reasonably well.