The Themes were all performed by members of the Ainur, Eru merely directed them. While Eru was displeased at the discord, he also firmly stated that even Melkor's discord was part of his design. Eru even ends the entire conflict in an instant when he wanted to.
(All following quotes are from the
Silmarillion, AINULINDALË).
"
In the midst of this strife, whereat the halls of Ilúvatar shook and a tremor ran out into the silences yet unmoved, Ilúvatar arose a third time, and his face was terrible to behold. Then he raised up both his hands, and in one chord, deeper than the Abyss, higher than the Firmament, piercing as the light of the eye of Ilúvatar, the Music ceased. "
That being said, Eru also stated that
"
Mighty are the Ainur, and mightiest among them is Melkor" which we currently have as evidence for them likely scaling to Eru, at least in dimensionality.
Of course, I personally still believe they should fully scale due to being
- Referred to as "mighty" by Eru, whereas the entire 4-D structure is but "thought" to him
- They resided in the same space/dimension as Eru. With this space being referred to as the "primary reality" compared to LOTR's "secondary reality"
- They can genuinely interact with Eru and converse with him, showing comparability in existence (even humanity, in some of the tales were Eru spoke to them, only vaguely perceived his voice, and they're specifically attuned to Eru in a unique way).
- The entirety of time and space were utterly beneath them, with their entry into it being seen as a "descent" where they had to restrain themselves to properly enter.
However, I'm undertaking my finals, so I have no time to push new revisions.
(There's also the Imgur thing and accepted revisions to change).
Edit: Actually, it's noted that the "
the halls of Ilúvatar shook and a tremor ran out into the silences yet unmoved". As the Halls of Ilúvatar refer to Eru's realm, that means the 5-D realm of Eru was shook by the conflict. That seems to be solid scaling proof.