I once did a comparison of the pure DI tone between Helix, Fractal, RND Shelford Channel and a RME UFX III. Nothing else in the chain, just guitar (full volume) - cable - device - DAW.
All are supposed to have 1M Ohms impendence but there were clearly audible differences between all units (kind of a surprise as I was exiting them to behave/sound similarly tbh). The Helix was the darker sounding one of them all. Not sure if this is where this "veil/darkness" is coming from. It would be interesting if other people could perform the same test (just strumming the same chords at the same dynamic and tempo).