Technically, nothing. But there are transformers and then there are transformers.
The Imp and other inexpensive DIs use an inexpensive transformer that works fine for most practical purposes. But (on an admittedly minuscule but measurable level) they don’t deal with bass as well as a Jensen or Lundahl transformer or a quality active DI, especially at hotter levels.
So there’s the reason in and of itself. It does fall under the splitting hairs column. I will say that I’ve noticed a troubling trend of smaller, not-so-talented sound providers buying inexpensive DI and mistreating them badly. I can’t recall a larger provider having Radials or Countryman’s that looked like they lost the battle with The Borg, although I’m certain they are out there.
I will say that a few years ago, I bought a $40 Livewire DI from Guitar Center for a practice rig DI and I was quite pleasantly surprised at how it was built and how good it sounded. I check things like bass pups, preamps, DIs, etc with a small Soundcraft mixer and my 1964 Audio in-ear monitors. The Livewire wasn’t a REDDI (not supposed to be!), but a Stingray and an active Jazz bass sounded pretty good with it. I meant to A/B it with a Bb basic Rapco, but never got around to it.
