Once I activated Seal C, I could not upgrade rare items in the Horodic Cube - probably because all items are now “Level 1”.
It’s clunky but it does still work.
The problem is you now have to know what level item you create and if you gamble at Kadala good luck… she sometimes (rarely but sometimes) gives level 61 items even when you’re level 70.
Basically just stick to crafting the “Sovereign” items… the ones right below the set/legendary and you’ll be good for the cube.
Remember, that although items show a required level of 1 for you, Kadala and drop items are still whatever level you might be, and if lower than 70 cannot be used for upgrades in the cube.
Do the items show as level 1 in Stash?
I hope so. The more annoying confusion this causes, the more chicanery I can laugh with.