Off beat, or not defined enough. But the review suggests the latter ("good timing"), and the formulation suggests that they're just not quite enough defined, and if you read more carefully you might find indications as to why...
"Underpinning the lower-end, the bass is full, although it lacks precision."
"The richness of the sound is compelling and smooth. It's only the vagueness of the bass that draws the line between good and great, and it's a pretty thin line at that."
A review is not a science; reviews here are well informed, cool to read, but when it comes down to chosing your gears, then what you need is your own ears. What I mean is just that, according to that "pretty thin line", you may not even hear that vagueness when you'll listen to them... unless you're only thinking about it. And on the other hand, it is possible too that you find their "mid range to die for" quite unnerving (for exemple)... In that sense, a review can do wrong, if it prejudices you...