What is the test for a motion for a new trial when newly-discovered evidence has the effect of impeaching a witness?

There is language in a number of our decisions which, at first blush, lends support to this argument. We have held that when the effect of newly-discovered evidence is merely to impeach a witness, a new trial is unwarranted. See United States v. Kulczyk, 931 F.2d 542, 549 (9th Cir.1991) ("[E]vidence that would merely impeach a witness cannot support a motion for a new trial."); United States v. Alexander, 695 F.2d 398, 402 (9th Cir.1982) ("Evidence which is merely impeaching is not sufficient to support a motion for new trial."). Here, however, the appellants argue the newly-discovered evidence would do more than merely impeach Duran; it would render his testimony useless. According to the appellants, with Duran's testimony eliminated, the remaining evidence against them is insufficient to support their convictions.