In the years between their debut and Coexist, the xx's sound took on a life of its own, thanks in large part to Drake's hit duet with Rihanna, "Take Care," which sampled Jamie Smith's collaboration with Gil Scott Heron, We're New Here.

That single embodied and popularized the xx's aesthetic to such a degree that on first listen, Coexist can sound like demos for a potential follow-up.

In turn, these songs lay the trio's R&B roots bare, with an extra emphasis on that last word: while second albums are where bands usually add more elements to their sound to keep things interesting, the xx go even darker and sparer than they were on their debut, which was pretty sparse and dark to begin with.

On the lovely album opener "Angels," elegantly serpentine guitars -- which recalled the Cure and Durutti Column on xx but are now entirely their own -- echo into spectral shadows, and fragments of beats and melodies hang unresolved in the air, surrounded by vast expanses of nothingness.

Any track here makes "Basic Space" or "Crystalised" sound like an Arcade Fire song by comparison, but the xx walk the fine line between minimalism and incomplete-sounding confidently.

All that silence throws the album's subtle sounds into sharp relief, highlighting the luminous keyboards and percussion on "Try" and "Tides"' undulating bassline more effectively than cranking up the volume on them would have.

Coexist's barely-there arrangements mean that singers Oliver Sim and Romy Madley Croft do most of the heavy lifting, but they're both more than capable of carrying the album's intimacy and humanity.

While their solo tracks show how much both of them have grown as singers since xx, Sim in particular has become more polished and emotive, sounding even sadder and more seductive on "Fiction" when he sighs "last night the world was beneath us." However, Coexist's heart lies in Sim and Madley Croft's duets.

The pair sounds more entwined than ever, and whether they're close together in Coexist's vast spaces, as on "Our Song"'s final glimmer of hope, or separated by them, as on "Chained," they share an intimacy that makes listeners feel like they're eavesdropping.

But while there's no question that this album is more accomplished than xx, it's also less accessible; these songs just aren't as immediate as the band's debut, and at times, these tales of love gone wrong or gone away threaten to become repetitive.

Still, there are so many moments of spine-tingling beauty, such as the way "Reunion" blends almost imperceptibly into "Sunset," or Madley Croft and Sim's shared sigh on "Unfold," that they're worth a little patience from the listener.

Coexist's exploration of isolation and intimacy is demanding and rewarding in its bold subtlety and eloquent simplicity.