Stop A/B testing two subject lines and hoping. Pre-write 15 falsifiable hypotheses (length, emoji, personalization, urgency, numbers, case, brackets, sender, preheader...) in a 15-row spreadsheet, design 2-4 cells per hypothesis, then let the data pick the winners — one row per send, not a multivariate stew. The 6 starter rows, the Klaviyo test setup, and why one row per send is the rule.
