Gar, welcome to Axiom. It would only be worth it if more power than the 709 in particular was capable of supplying was needed. It's highly unlikely, especially with the sub carrying the low bass burden, that more maximum power output is actually required. At typical comfortably loud average output levels the Axioms use about 1 watt. Brief peaks require much more of course, but not likely more than the 100 plus watts which the receiver can supply at inaudibly low distortion levels.
Ample headroom is already highly likely present. Buying a separate amplifier which would provide yet more headroom isn't economically sensible; unused headroom is simply that: unused.
