I would recommend you look at the ML website
for information about the impedance variations by frequency of the different models. You might also try the comments of Martin Logan reviewers in the archives
"Too much power" should not really be an issue (I drove mine with 250-400W amps) unless you are looking to do outdoor parties for a hundred people (in which case, get some old JBLs). The amp's ability to drive the speakers could be an issue. MLs are not terribly easy on amps, and if set up correctly (Placement! Placement! Placement!), they will teach you things about your amp you didn't know before. I personally decided I liked tubes in the chain with them.
FWIW, I believe the impedance on SL3 actually drops to a very low level at the top end of the audible (or inaudible) range - 1 or 1.2 ohm for the SL3s.