What's the intuition for PTAS reductions instead of approximation preserving reductions or something of that ilk? I'd expect APX-Hardness to mean something like finding a constant factor approximation for this problem is as hard as finding a constant factor approximation to any problem in APX but having just a PTAS reduction from A to B isn't strong enough to imply a constant factor approx to B can be converted into a constant factor approx of A. 
Why are PTAS-Reductions used to establish APX-Hardness?
complexity theory;reductions;approximation