I read your ad. Let me ask a question.
Do you have sane working hours? Do you encourage workers to take weekends off and take vacation? Are they really disconnected on the weekends and on vacation?
If you do have a good work/life balance culture at your company, stress that in your ad. Stress it several times.
Unfortunately many developers with years of experience read “startup” and translate that into “death march.” Since they are senior, they have experienced burn out, and know what that does to your health. They also know how long hours is a management anti-pattern.
If Durran Jordan is right and senior developers are happy where they currently are, then one must mark strongly that they will be happy in a new job.