You say they both failed at the edge of the reinforcement by the valve. This prompts me to ask: how is your inflation technique? Are you inadvertently pushing the valve in (or pulling it out) too many times as you go to inflate them before each ride? I have found that when I run the pink latex Vittoria tubes I will develop leaks in that same general vicinity and the failure is caused by my own trauma I am causing by my pump head.
I came up with my own system of keeping the unthreaded valve stems from moving:
