The Borodin Quartet has much to celebrate these days: the 80th birthday of founding cellist Valentin Berlinsky, the 60th anniversary of Berlinsky’s quartet, the 50th anniversary of the group’s taking the name “Borodin,” and a new association with the new British boutique label Onyx, which provides a serious recording outlet for major artists no longer under contract to major labels.

If you remember the ferocity and sometimes steely tone of the Borodin Quartet from the 1950s into the ’70s, and have missed the ensemble’s more recent recordings, you may be surprised by the relaxation and warmth of this new recording. Partly that’s a function of the repertoire, which is generally mellow. In the group’s signature work, Borodin’s String Quartet No. 2, the playing is sweet and unpressured, except that the violins can occasionally sound astringent in the highest-lying passages and there’s a spooky rawness to the group’s use of straight tone in the outer movements. Compared to the ensemble’s 1980s Melodiya recording of this work, which showed up in the States on EMI, this version is just a bit slower and smoother overall. Not undercharacterized, though; in the famous Nocturne, Berlinsky’s opening phrasing is much freer now, even slightly idiosyncratic and recitativelike.

The remainder of the disc shares all these qualities. Even the Webern movement is a Romantic effusion in slow tempo, a student piece that owes much to Schoenberg’s Transfigured Night. It’s a very attractive hour of quiet, mostly Slavic listening.