London Haydn Quartet Brings Its Gut Strings to the Morgan London Haydn Quartet Eric Hoeprich, playing the basset horn, joined the ensemble during its concert at the Morgan Library & Museum. Like a glass of prosecco before a more robust wine, Haydn's sunny string quartets are often served as an appetizer to a weightier work. But the London Haydn Quartet has sought to elevate the status of Haydn's some 75 string quartets, exploring the many neglected gems of his vast catalog. A period ensemble that plays on gut strings with classical bows, the group (the violinists Catherine Manson and Michael Gurevich, the violist James Boyd and the cellist Pierre Doumenge) offered a revelatory performance on Tuesday evening at the Morgan Library & Museum's Gilder Lehrman Hall, presented in tandem with the Boston Early Music Festival. The ensemble opened the program with an exquisitely rendered interpretation of Haydn's infrequently performed String Quartet in B flat (Op. 50, No. 1), written in 1787 and dedicated to the cello-playing king of Prussia. The musicians highlighted the mysterious and brooding character of the first movement, which opens with a questioning cello motif. The earthy, warm sounds of the gut strings blended beautifully throughout. The quartet's deeply committed music making seemed indicative of a longstanding and nourishing partnership, as the physically demonstrative players were acutely alert to their colleagues" every gesture. They brought a more full-bodied sound to the String Quartet in D (Op. 18, No. 3) by Beethoven, who was profoundly influenced by Haydn, often called the father of the string quartet. Like the opening Haydn piece, the Beethoven quartet begins with a few delicate, harmonically ambiguous measures. The work also pays homage to Haydn with the witty, unexpected ending of the concluding Presto, played here with passion and verve. The musicians imbued both works with myriad details of shading and contrast, and beautifully calibrated phrasing. Barring a few tiny slips, they played with impressive intonation throughout the concert. They were joined by the clarinetist Eric Hoeprich for Mozart's Clarinet Quintet in A (K. 581). During a preconcert lecture, Mr. Hoeprich discussed the main differences between the basset horn, the period instrument he used here, and its modern counterpart. He demonstrated how a player must execute trills differently to render them effectively on the earlier instrument, and pointed out other qualities like the historical instrument's lower range. Mr. Hoeprich's rounded, mellow tone meshed alluringly with the warm sound of the gut strings. The second movement Larghetto, one of Mozart's most sublime creations, sounded hauntingly lovely.