A/B testing subject lines vs. A/B testing the list

A/B testing subject lines vs. A/B testing the list
Popular advice: "Split-test subject lines until you find the winning open rate."
Myth. You're optimizing the cheapest variable in the funnel. Subject A versus B might swing opens a few points, but the list you chose swings replies tenfold — wrong creators don't answer a perfect subject line. Splitting subjects on a bad segment is rearranging deck chairs. Test who you send to before you test what the envelope says. It's not the subject line that's underperforming, it's the audience behind it.