I think I know what he means. There's a tubey warmth in the basic Crack circuit that's very appealing. What it lacks in resolution and bass "quickness" is matched by an old-fashioned "syrupy" quality (thick and sweet) and, with some headphones, an addictive sense of space and dimensionality. I don't know if it's the cathode follower, the simple circuit, or both, but it's very engaging.
I built a Mainline and found it a bit "clinical" in comparison. The resolution was clearly better with the Mainline, but the "magic" from the Crack was missing. At least to my ears with my headphones (Senn HD800 and Audeze LCD2). Perhaps some of the technical drawbacks of the Crack (which result in a slowish and syrupy sound) aren't really drawbacks but highlights for some listeners.
More than most BH products, I think the Crack harkens back to the classic tube sound of the '50s and early '60s. Just imagine replacing its diodes with a tube rectifier! Anathema for the BH team, but it's a sound some people like.