Well, I find the term 'laid back' to be somewhat ambiguous. The Senns 600/650 or Audeze's are often said to be quite laid back. And this is usually said in contrast to 'fun' headphones. But what does that mean? People conflate a variety of things it seems: sometimes it means it is a neutral sound, sometimes it means the profile in general is dark (i.e. not much emphasis on the low/high frequences, even if they are mid forward). Sometimes it means the sound is warm and smooth. From what I've been able to muster, the X1's have a pretty prominent emphasis on the lower frequencies, with big bass response, and some sparkle on the treble as well. The mids are a bit, though not dramatically recessed. Smooth all the way, with no sibilance.
They might not be as bright as the Grados, and so might appear relatively 'smoother' and balanced compared to those. But they are much more playful at the ends than, say, the Senns, and so are not a neutral can, nor certainly a dark one.
I suppose that's what people mean that this is a 'jack-of-all-trades' can: it can be described as laid back (relative to the Grados, or even something like the he-400 which is said to be a bit rough higher up), but also as fun (relative to the Senns, or something with a more mid-centric, less bassy, smoother and and darker).