The secret here is that the additional '0.20 - 0.25 attacks per second' applies to BOTH weapons!
This means that if your offhand is a fist weapon at 1.40 attacks per second, it will improve to 1.6x just by equipping an Echoing Fury. (14%-18% increase in offhand damage, plus other speed benefits)
If its a mace/spear at 1.20aps = 17%-21% increase
This mace in my profile was a 7k dps increase over any other upgrades I could find at an equivalent price on the current market. (researched in the last hour)