Set items will never spawn ethereal.
Your statement about the indestructible mod doesn't make sense, though, since items that have the indestructible mod also can never spawn ethereal*. So even if Aldur's Advance was a unique item, it couldn't be ethereal, because it's indestructible.
The indestructible mod on those boots means they never have to be repaired. Nothing more. Nothing less.
* The only exceptions are the items Ethereal Edge, Shadow Killer and Ghostflame, which have both the indestructible mod and the always-ethereal mod.
Your statement about the indestructible mod doesn't make sense, though, since items that have the indestructible mod also can never spawn ethereal*. So even if Aldur's Advance was a unique item, it couldn't be ethereal, because it's indestructible.
The indestructible mod on those boots means they never have to be repaired. Nothing more. Nothing less.
* The only exceptions are the items Ethereal Edge, Shadow Killer and Ghostflame, which have both the indestructible mod and the always-ethereal mod.