I don't know if I'd call those passives, because you can launch them at enemies. Seeker swarms are the only way for a collector player to detonate his own BE's primed by DS and DC. And it can be highly effective to throw a DS, tag something with DC, then start launching swarms at the DC target and everything the sphere passes. And then detonate the sphere to take out whatever survived the BE's. And there's a Juggernaut build that uses Siege Pulse as a primary attack, revolving around detonating explosions primed by ammo mods.
On the volus vanguard, Biotic Orbs can be considered a passive, use to speed up the use of Charge and Shield Boost. But on the adept, they're kind of like the collectors seekers swarms, in that they can be used to detonate BE's primed by Stasis. And none of this is taking into account detonating explosions primed by other players.
Of the ones mentioned so far, I think I'll go with AH. Though Bloodlust is close behind.
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