According to Joe Buchalter, president of Metric Halo: "The ULN-2 is an incredibly good mic pre, but sometimes you want something that isn't quite 'so pristine' - a certain amount of low-order harmonic distortion has a way of adding depth and sheen to everything that passes through it, thus the high regard applied to classic transformer coupled preamps.
"With the transformer option installed, the ULN-2 acquires that classic character. A second benefit is that the transformer completely electrically isolates the preamp from the source, raising the common mode rejection above its already excellent performance, and providing a measure of confidence when dealing with the unknowns of location sound."
(Jim Evans)
