By good angels tenanted, (2)
These angels make a quick cameo at the start of this poem, but they don't really come back. We often associate angels with some kind of religious belief. Usually they are a part of some bigger spiritual order—servants of God, for example. In this case, though, their presence seems kind of vague. They hang out and lend some of their generally positive vibe to the valley and the palace, but we don't learn much about them. Mostly, they seem to contribute to the dream-like beauty of the opening stanzas.
Spirits moving musically, (19)
These spirits represent the orderly, stable condition of the human body and mind. In the opening stanzas they move in time to the music, round and round the throne of the king, Thought. Beyond that, we don't learn anything about who these spirits are, or what they are doing. It seems to us, though, that the palace is just as "haunted" here as it is at the end. So it's not really being filled with spirits that's the problem at the end. It's the kind of spirits we're talking about that counts.
A troop of Echoes, whose sweet duty (29)
These cheerful critters flowing out of the palace represent the speech that comes out of the head while it's still in good shape. The hint that tips us off to that is the fact that they are named after Echo. She was a Greek nymph who could not speak for herself, but only "echo" the words of others. So these particular spirits aren't here to come up with their own ideas, but to take direction from the head.