diablo-3
I just loaded up the new patch and there doesn't seem to be much direction on how to actually acquire this key new item. Zultan Kulle talked about it briefly, and then I had no quests indicating where to go next.
How exactly do I acquire Kanai's Cube?
As you have said, you are able to place a Puzzle Ring inside Kanai's cube, hit transmute and a portal to the Treasure Realm will be opened. Likewise if you place a Bovine Bardiche, a legendary Polearm, in the cube and hit transmute, a portal that definitely doesn't lead to a secret cow level, definitely won't open, wink. These are the only currently known secret cube recipes.
I can now attest first hand that set items are possible, got a Uliana's Strength upgrading a rare shoulder piece with my monk.
With these 2 reddit's threads confirming this also, I'm confident enough to say that the items upgraded will all be for the class they are being upgraded with. Else, set items would not be 100% sure of being for the class upgrading the items which would seem strange.
That is also consistent with crafted items that have stats for the class crafting them.
But, the class-only items will roll with their class stats. I tested with a voodoo mask and got 
Best Answer
Just teleport to the new zone, Sescheron Ruins (act III, Top Right corner of map) and explore. I found it in the Elder Sanctum map area.