John,
Thank you so much for reminding me of that apothegm, which I had not remembered, but was the sliver stuck in my mind. You led me to
this wonderful exploration of the subject by Clive James (citing Eliot, Yeats, Shakespeare and Larkin, among others).
The mature Whitman and Rumi seem rather happy to me. I consider them both great. Perhaps Blake, though his happiness is of a rare (semi-cracked?) variety?
M