I asked about the AM882/EM806 difference a week or so ago. The answer is that the AM882 is an older version, now obsoleted by the EM806 which is in some unknown way "better" - but I'm not sure quite why or how! Zapp only have one of the AM882s left and as it's the same price as the newer model, it might be there for a while (unless someone needs a replacement, maybe). Don't know about the DM870.
As far as current is concerned, are there some measurement problems given that the output is generally PWM and this might not correctly read on a digital meter? Just thinking out loud. Ditto power measurements - need to take phase difference between voltage and current into account with a highly inductive load.