It was the Outback I was looking at.  They have a tool that calculates Voc at various temperatures for various panels to see if the MX60/FM60/FM80 can handle it.  

The link is:  http://www.outbackpower.com/forum/viewtopic.php?f=7&t=2900

Then click on "String Sizing Tool".

The record low temp where I'll be installing the system is -25 degrees F, which made the Voc too high for these controllers.  

I have found some 180W and 195W panels with lower voltages, so I can string three together in series and still fall well within the limits of the controllers even at very cold temps.