Mozart’s 1785 dedication of six quartets to Haydn reads: ‘May it therefore please you to receive them kindly and to be their Father, Guide and Friend.’ Respect is clear; but when the 96-bar Andante con moto of K428 offers a strong reminder of the 96-bar Affettuoso e sostenuto of Haydn’s Op 20 No 1, also in A flat and in compound time, affection becomes implicit too. Cuarteto Casals don’t stint on tender warmth, ensemble expertly balanced and chorale-like in sonority. Yet clarity remains uppermost. An un coagulated sound is an ever-present aspect of this ensemble’s style. So is an emotional and intellectual dimension, probed through trenchant attack, elastic lines, ductile phrases and a wide dynamic range.

Not a word on ‘authenticity’ or ‘historically informed’ practices. Rather a scant regard for superficial niceties. Pick K465 to represent the Casals’ approach, the Adagio introduction unequivocal in explaining why these 22 bars of startling false relations – A flat/A, G flat/G – and grating progressions like the chromatic sforzando clashes of F sharp/G between cello and viola raised a ruckus in its day. Reach the development of the main Allegro and the tense, driving power of the playing lifts the music to another level of interpretative penetration. Most arresting of all is the slow movement, for these musicians a sequence of pain and abraded nerve-ends behind a smokescreen of Andante cantabile. Cuarteto Casals shatter a glass ceiling of historic inhibitions and camouflage nothing. Enshrined herein is a rare order of musicianship.