For starters, it is far more important that your center speaker match your mains than that it match your surounds. That doesn't mean the ML won't work, but a Polk center would be a much better bet.
My guess is that your AVR wouldn't choke on a single 4 ohm speaker. But you'd have to try it and see. In any event, buying a new amp to accommodate a center speaker that might not be a good match for your system anyway is really bass-ackwards.
If it were me, I'd sell the center and the Emotiva amp (you really don't need it) and put the proceeds toward a Polk center and surrounds, or else a whole new 5.1 system.
If you can't explain how it works, you can't say it doesn't.—The High-End Creed