The life-blood of rhymed translation is this, that
a good poem shall not be turned into a bad one.
The only true motive for putting poetry into a fresh
language must be to endow a fresh nation, as far as
possible, with one more possession of beauty. Poetry
not being an exact science, literality of rendering is
altogether secondary to this chief aim. I say literality,
not fidelity, which is by no means the same thing.
When literality can be combined with what is thus
the primary condition of success, the translator is
fortunate, and must strive his utmost to unite them;
when such object can only be attained by paraphrase,
that is his only path.

--Dante Gabriel Rossetti

Rossetti's translations of Calvacanti's are better than Pound's. That single principle, that a good poem cannot be translated by a bad one, seems essential. It would seem to establish a criterion of aesthetic fidelity. The original is a good poem, the translation is not. Therefore it cannot be a good translation, even if it is a faithful and literal translation in all other respects. That is the implication, at least.

Another principle would be to try the literal first, and then move to paraphrase if that doesn't work. Or, in other words, take advantage of the places where the literal does in fact work.

Of course, there's no convincing someone that a translation is a "bad poem," when that person wants to say that the translation is adequate. I can convince someone that the translation is faulty on semantic grounds, if we agree on the meanings of words, but not on aesthetic grounds, if we disagree about where beauty lies.