DY: The Shevet HaLeivi’s Shittah is that anything made to sound good inherently counts as not a capella.
Rav Moshe’s music T’shuvah is OC 1:167, I believe. But he does say in there that Sefira is Assur even a capella, though the Minhag to me Meikel on a capella kinda makes sense based on him (emphasis on the kinda).