In general I would say the closest is Squanch. They aren't similar, but they are comparable. Main difference is Damu is excellent, Squanch is better. They both taste like the bitter yanqona they are. Oh, and with Squanch I get euphoria on the first shell then profound cosmic mellow afterwards but with Damu I get euphoria with every shell, plus the profound mellow. OTOH, the euphoria with Squanch is even more euphoric, and melty, and the following mellow is even more profound. And there's a playing with fire/locking into some essential flow of energy emerging from deep in the bowels of the earth, full of magma and laughter, that you get with Squanch and don't get with Damu. Damu is very civilized, sophisticated, even. Squanch is more elemental. You can tone that down a bit, I like to mix it with Kula or Tongan Family Reserve. Haven't tried it with Pouni Ono or that Vava'u I got yet, but I'm guessing there's similar effect, just as balanced, but with a slightly more dignified euphoria. Damu I would serve to people I wanted to impress, like my boss. Squanch is for family and close friends and soulmates.
Disqualifier: I gave my son a pound of each. I should ask his opinion. He loves both, of course. But he only went ape over Squanch.