For those that haven't used it yet, transfiguring an item with the cube can do some really fun things. Maybe it'll add a new stat line, maybe it'll turn one of the existing stat lines into a GA, WHO KNOWS?!?!
But one thing it does a lot not matter what your stat result is: makes the item unmodifiable. That means no tempers, no masterworks, no enchanting, you can't even put a gem slot in it. I originally thought it just meant no more crazy changes from the cube, but it means literally everything.
Make sure you're really ready for it to be the final upgrade before you transfigure. I just bricked a 2GA that I had built up from a blue, and I only have a single GA item to my name as it is so that was a lot of upgrade that just fizzled into a couple baleful fragments and a forgotten soul, which doesn't even come close to what I spent building it in the first place. .