Jason pointed me to some discussions he had on the MyKBO.net Facebook page about the set. It appears that the three "subsets" are short printed. There's one poster who said he'd opened 10 boxes (20 packs of six cards each) and still did not have a complete set.
The boxes are cheap enough ($22) that I ordered two boxes so I'll see for myself in two to three weeks. Maybe I can do some trading to complete the set.