An unhelpful non-canonical answer - but in The Dying Earth RPG I'm pretty sure IOUN stones require attunement, which is presumably based on what Vance wrote in his stories.DMG does not mention attunement but SRD says Attunement. Which is officially true?
....to quote the Great Buddha, Keanu, .... Woah ....Personally I think attunement is fine, the limit of 3 is like you know really limiting. Use attunement but drop the number limit completely.
Now the headband of intellect becomes the most valuable item in the world.Here is a thought: maximum attunment slots = 3 +/- Int modifier.