@BlackHeartV - thanks a lot for the info - I hadn’t realised there was a separate loot pool for Pearls. I guess I can stop being annoyed at all the orange drops from tubbies, since they’re not stopping the Pearls from dropping. Maybe getting to OP8, then farming. would be “easier” than a L72 farm then… and I’d been putting off on OP levels because of the difficulty! 
@shure - about getting tubbies to appear, I’m pretty sure if you visit Hyperius’ gate to activate 4-player difficulty, then do the first Creature Slaughter round repeatedly, you’ll see about the same tubby rate as I did - about 2 tubbies every 3 runs. It is a really reliable way to spawn them as you can see from the notes I pasted. and you don’t have to quit-save between runs. The only trick (apart from coping with 4-player mode solo) is to make sure not to kill all the enemies in the last round. Usually there’s 3 or 4 spiderlings at the end. I mostly just ran away from them, jumped up against a wall and rocketed myself in the face, though one time the stupid spiderlings jumped after me as I did it, and I accidentally took them out.
