Finding (calculating?) speaker and amp impedance

I'm starting to understand why speakers with high efficiency and perhaps more importantly a flat impedance curve work well with tube amps.

If not published by the vendor/manufacturer, is there a way to determine the impedance curve from the specs that are provided?

Also, I have a pair of Magnepans that need high current amplification.

Is there a way to determine the current of an amplifier from vendor/manufacturer provided specs?

Thanks everyone!